U.S. patent application number 09/924433 was filed with the patent office on 2002-01-24 for selective deposition modeling method and apparatus for forming three-dimensional objects and supports.
This patent application is currently assigned to 3D Systems, Inc.. Invention is credited to Almquist, Thomas A., Bedal, Bryan J.L., Earl, Jocelyn M., Fedchenko, Richard P., Hull, Charles W., Kerekes, Thomas A., Leyden, Richard N., Lockard, Michael S., Merot, Christian M., Pang, Thomas H., Smalley, Dennis R., That, Dinh Ton, Thayer, Jeffrey S..
Application Number | 20020008335 09/924433 |
Document ID | / |
Family ID | 27064600 |
Filed Date | 2002-01-24 |
United States Patent
Application |
20020008335 |
Kind Code |
A1 |
Leyden, Richard N. ; et
al. |
January 24, 2002 |
Selective deposition modeling method and apparatus for forming
three-dimensional objects and supports
Abstract
A variety of support structures and build styles for use in
Rapid Prototyping and Manufacturing systems are described wherein
particular emphasis is given to Thermal Stereolithography, Fused
Deposition Modeling, and Selective Deposition Modeling systems, and
wherein a 3D modeling system is presented which uses multijet
dispensing and a single material for both object and support
formation.
Inventors: |
Leyden, Richard N.;
(Topanga, CA) ; Thayer, Jeffrey S.; (Montara,
CA) ; Bedal, Bryan J.L.; (Palmdale, CA) ;
Almquist, Thomas A.; (San Gabriel, CA) ; Hull,
Charles W.; (Santa Clarita, CA) ; Earl, Jocelyn
M.; (Old Headington, GB) ; Kerekes, Thomas A.;
(Calabasas, CA) ; Smalley, Dennis R.; (Newhall,
CA) ; Merot, Christian M.; (Saugus, CA) ;
Fedchenko, Richard P.; (Saugus, CA) ; Lockard,
Michael S.; (Grand Junction, CO) ; Pang, Thomas
H.; (Castaic, CA) ; That, Dinh Ton; (Irvine,
CA) |
Correspondence
Address: |
Ralph D'Alessandro
3D System, Inc.
26081 Avenue Hall
Valencia
CA
91355
US
|
Assignee: |
3D Systems, Inc.
|
Family ID: |
27064600 |
Appl. No.: |
09/924433 |
Filed: |
August 6, 2001 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
09924433 |
Aug 6, 2001 |
|
|
|
09252512 |
Feb 18, 1999 |
|
|
|
6270335 |
|
|
|
|
09252512 |
Feb 18, 1999 |
|
|
|
08722335 |
Sep 27, 1996 |
|
|
|
08722335 |
Sep 27, 1996 |
|
|
|
08534813 |
Sep 27, 1995 |
|
|
|
Current U.S.
Class: |
264/494 ;
264/308; 425/174.4; 425/375 |
Current CPC
Class: |
B33Y 10/00 20141201;
B29C 41/36 20130101; B33Y 50/02 20141201; B29C 64/40 20170801; B33Y
40/00 20141201; B29C 64/106 20170801; B29C 41/12 20130101; B29C
64/112 20170801; B33Y 30/00 20141201 |
Class at
Publication: |
264/494 ;
425/174.4; 425/375; 264/308 |
International
Class: |
B29C 035/08; B29C
041/08 |
Claims
What is claimed is:
102. A method of creating a three-dimensional object by depositing
a build material on a working surface from a plurality of
dispensing orifices in a print head in a rapid prototyping system
to form multiple layers of a three-dimensional object, the method
comprising the steps of: establishing a relative position between
the print head and the working surface; selectively dispensing the
build material from the dispensing orifices of the print head while
providing relative movement between the print head and working
surface along a first path; re-positioning the relative position
between the print head and the working surface in a direction along
a second path; repeating the step of selectively dispensing the
build material; and repeating the steps of re-positioning and
selectively dispensing to form the multiple layers of the
three-dimensional object.
103. The method of claim 102 wherein the build material is curable
upon exposure to radiation, the method further comprising the step
of: exposing the dispensed build material to radiation to cure the
layers of the three-dimensional object.
104. The method of claim 103 wherein the build material includes a
photopolymer that is curable upon exposure to ultraviolet
radiation.
105. The method of claim 103 wherein the step of exposing the
dispensed build material to radiation is done for each layer after
each layer has been dispensed.
106. A method of claim 102 wherein after a layer is formed the
working surface is then established on the just formed layer for
forming a new layer, the method further comprising the step of:
re-establishing the relative position between the print head and
the new working surface on the just formed layer.
107. The method of claim 106 wherein the step of re-establishing
the relative position is achieved by moving the print head to
compensate for the thickness of the just formed layer.
108. The method of claim 106 wherein the step of re-establishing
the relative position is achieved by moving a build platform to
compensate for the thickness of the just formed layer, the
three-dimensional object being formed on the build platform.
109. A method of claim 102 wherein the plurality of orifices
establish an elongated dispensing pattern having an axis extending
generally along the second path.
110. A method of claim 109 wherein the orifices are equally spaced
at a given distance measured in a direction along the second path,
and wherein the step of re-positioning the relative position
between the print head and the working surface in the second path
in done by shifting the relative position in equal increments of
the given distance.
111. The method of claim 109 wherein the orifices are equally
spaced at a given distance measured in a direction along the second
path, the method further comprising the step of: establishing a
saber angle between the axis of the elongated dispensing pattern
and the second path in order to adjust the given distance measured
to achieve a desired resolution.
112. The method of claim 102 further comprising the step of:
controlling the dispensing of the orifices of the print head to
achieve a uniform thickness of build material in the layers.
113. The method of claim 102 further comprising the step of:
indexing the relative position between the print head and the
working surface along the second path an amount at least equal to
the width between the outermost orifices of the print head measured
in a direction along the second path.
114. The method of claim 113 wherein the step of indexing is
executed as needed to form any layer of the three-dimensional
object.
115. The method of claim 113 wherein the step of selectively
dispensing the build material while providing relative movement
along the first path is completed at least once prior to the step
of indexing the relative position between the print head and the
working surface along the second path.
116. The method of claim 115 wherein the step of selectively
dispensing the build material while providing relative movement
along the first path is completed eight times prior to the step of
indexing the relative position between the print head and the
working surface along the second path.
117. The method of claim 102 further comprising a plurality of
print heads for dispensing the material on the working surface to
form the three-dimensional object.
118. The method of claim 117 wherein the print heads are arrayed
end to end along the second path.
119. The method of claim 117 wherein the print heads are arrayed
back to back along the first path.
120. The method of claim 117 wherein the print heads are offset
from one another in the second path.
121. The method of claim 102 wherein each dispensing orifice
selectively dispenses the build material along a raster line, the
raster lines comprising a plurality of raster lines.
122. The method of claim 121 wherein the step of re-positioning the
relative position between the print head and the working surface in
the second path causes each dispensing orifice to dispense the
build material along an alternate raster line of the plurality of
raster lines to compensate for orifices that are not dispensing
correctly.
123. The method of claim 121 wherein the step of re-positioning the
relative position between the print head and the working surface in
the second path causes each dispensing orifice to dispense the
build material along a different raster line that is not part of
the plurality of raster lines to compensate for orifices that are
not dispensing correctly.
124. The method of claim 102 further comprising the step of:
detecting optically any misfiring orifices.
125. The method of claim 102 further comprising the step of:
calibrating the plurality of orifices.
126. The method of claim 125 wherein the step of calibrating the
orifices comprises: depositing a layer by selectively dispensing
build material from all of the dispensing orifices; detecting
optically any misfiring orifices; compensating for any misfiring
orifices detected.
127. The method of claim 126 wherein a test pattern is formed when
depositing the layer of the calibration step.
128. The method of claim 126 wherein the step of compensating for
any misfiring orifices comprises: putting the misfiring orifices
through a reactivation routine.
129. The method of claim 126 wherein the step of compensating for
any misfiring orifices is accomplished by overprinting.
130. The method of claim 102 further comprising: compensating for
any misfiring orifices when forming each layer to achieve a uniform
thickness for each layer.
131. The method of claim 102 wherein the step of re-positioning the
relative position between the print head and the working surface in
the second path is achieved by moving the print head,
132. The method of claim 102 wherein the step of re-positioning the
relative position between the print head and the working surface in
the second path is achieved by moving a build platform, the build
platform supporting the layers of the three-dimensional object
being formed and therein controlling the location of the working
surface.
133. The method of claim 132 wherein the step of re-positioning the
relative position between the print head and the working surface is
further achieved by moving the print head along with the build
platform.
134. The method of claim 102 wherein the step of re-positioning the
relative position between the print head and working surface is
further accomplished by: rotating respectively the print head with
the working surface in order to alter the direction of the first
path.
135. The method of claim 134 wherein the print head is rotated with
respect to the working surface.
136. The method of claim 134 wherein a build platform is rotated
with respect to the print head, the build platform supporting the
just formed layers of the three-dimensional object and therein the
location of the working surface.
137. A method of forming a three-dimensional object layer by layer
in successive layers of material are selectively formed in accord
with data defining the object, wherein each layer is formed by
selective deposition of a flowable material comprising the
following steps: moving a printhead in a first scanning direction,
the printhead having a plurality of orifices extending at an angle
to the first direction, selectively depositing material from the
orifices while moving the printhead in the first direction along a
first scan path; moving the printhead in a second direction angled
to the first direction; and selectively depositing material from
the orifices while moving the printhead in the first direction
along a second scan path offset from the first scan path.
138. The method according to claim 137 further comprising the
second scan path overlapping the first scan path.
139. The method according to claim 137 further comprising the
material being a flowable material curable by exposure to
radiation.
140. The method according to claim 137 further comprising the
orifices extending transverse to the first direction.
141. The method according to claim 139 wherein the material is
ultra-violet (UV) curable.
142. The method according to claim 139 wherein the material is a
photopolymer.
143. The method according to claim 142 wherein the photopolymer
comprises a photoinitiator.
144. The method according to claim 143 further comprising exposing
each deposited layer to radiation prior to depositing the next
layer.
145. The method according to claim 137 wherein material is not
deposited while the printhead is moving in the second
direction.
146. The method according to claim 137 further comprising using a
plurality of printheads to dispense the material.
147. The method according to claim 137 further comprising the
second direction being transverse to the first scanning
direction.
148. The method according to claim 146 further comprising the
plurality of printheads are arrayed end to end extending in the
second direction.
149. The method according to claim 146 further comprising the
plurality of printheads are arrayed back to back in the first
direction.
150. The method according to claim 149 further comprising the
plurality of printheads being offset from one another in the second
direction.
151. The method of claim 137 wherein the moving of the printhead in
the first scanning direction is along a plurality of parallel
raster lines.
152. The method of claim 151 wherein the moving of the printhead in
the second direction at an angle to the first direction is a
distance equal to at least one raster line.
153. Apparatus for forming a three-dimensional object in which
successive layers of material are selectively formed in accordance
with data defining the object, comprising: a printhead, means for
moving the printhead in a first scan direction and in a second
direction angled to said first direction, the printhead having a
plurality of orifices extending in a direction angled to the first
direction, the orifices being selectively activatable in accord
with data supplied to the printhead to dispense material, and
control means coupled to the means for moving to control movement
thereof and to the printhead to control selective activation
thereof.
154. The apparatus according to claim 153 wherein the control means
is programmed to cause the means for moving to cause a) movement of
the printhead in the first direction along a first scan path while
selectively activating the printhead to dispense material, b)
movement of the printhead in the second direction, and c) movement
of the printhead in the first direction along a second scan path
offset from said first scan patch while selectively activating the
printhead.
155. The apparatus according to claim 154 wherein the second scan
path overlaps the first scan path.
156. The apparatus according to claim 153 further comprising a
source of flowable material connected to the printhead for
dispensing to permit selective depositing of the flowable material
from the orifices.
157. The apparatus according to claim 156 wherein the flowable
material is a radiation-curable material.
158. The apparatus according to claim 157 in which said flowable
material is curable by exposure to ultra-violet (UV) radiation.
159. The apparatus according to claim 158 wherein the flowable
material is a photopolymer.
160. The apparatus according to claim 159 wherein the photopolymer
comprises a photo-initiator.
161. The apparatus according to claim 157 further comprising means
for delivering radiation for curing the flowable material
selectively deposited in each layer.
162. The apparatus according to claim 153 wherein the means for
moving the printhead moves the printhead in the second direction
transverse to the first scan direction.
163. The apparatus according to claim 153 further comprising using
a plurality of printheads to dispense the material.
164. The method according to claim 163 further comprising the
plurality of printheads are arrayed end to end extending in the
second direction.
165. The method according to claim 163 further comprising the
plurality of printheads are arrayed back to back in the first
direction.
166. The method according to claim 165 further comprising the
plurality of printheads being offset from one another in the second
direction.
167. The apparatus according to claim 153 further comprising the
plurality of orifices extend transversely to the first scan
direction.
Description
[0001] This application is a continuation of prior U.S. application
Ser. No. 09/252,512, filed Feb. 18, 1999, which is a divisional of
U.S. application Ser. No. 08/722,335, filed Sep. 27, 1996, now
abandoned, which is a continuation-in part of U.S. application Ser.
No. 08/534,813, filed Sep. 27, 1995, now abandoned.
[0002] 1. Field of the Invention
[0003] This invention relates to techniques for forming
three-dimensional (3D) objects and supporting those objects during
formation; more particularly, it relates to techniques for use in
Rapid Prototyping and Manufacturing (RP&M) Systems; and most
particularly to building and supporting methods and apparatus for
use in a Thermal Stereolithography (TSL) system, Fused Deposition
Modeling (FDM) system, or other Selective Deposition Modeling (SDM)
system.
[0004] 2. Background Information
[0005] Various approaches to automated or semi-automated
three-dimensional object production or Rapid Prototyping &
Manufacturing have become available in recent years, characterized
in that each proceeds by building up 3D objects from 3D computer
data descriptive of the objects in an additive manner from a
plurality of formed and adhered laminae. These laminae are
sometimes called object cross-sections, layers of structure, object
layers, layers of the object, or simply layers (if the context
makes it clear that solidified structure of appropriate shape is
being referred to). Each lamina represents a cross-section of the
three-dimensional object. Typically lamina are formed and adhered
to a stack of previously formed and adhered laminae. In some
RP&M technologies, techniques have been proposed which deviate
from a strict layer-by-layer build up process wherein only a
portion of an initial lamina is formed and prior to the formation
of the remaining portion(s) of the initial lamina, at least one
subsequent lamina is at least partially formed.
[0006] According to one such approach, a three-dimensional object
is built up by applying successive layers of unsolidified, flowable
material to a working surface, and then selectively exposing the
layers to synergistic stimulation in desired patterns, causing the
layers to selectively harden into object laminae which adhere to
previously-formed object laminae. In this approach, material is
applied to the working surface both to areas which will not become
part of an object lamina, and to areas which will become part of an
object lamina. Typical of this approach is Stereolithography (SL),
as described in U.S. Pat. No. 4,575,330, to Hull. According to one
embodiment of Stereolithography, the synergistic stimulation is
radiation from a UV laser, and the material is a photopolymer.
Another example of this approach is Selective Laser Sintering
(SLS), as described in U.S. Pat. No. 4,863,538, to Deckard, in
which the synergistic stimulation is IR radiation from a CO.sub.2
laser and the material is a sinterable powder. This first approach
may be termed photo-based stereolithography. A third example is
Three-Dimensional Printing (3DP) and Direct Shell Production
Casting (DSPC), as described in U.S. Pat. Nos. 5,340,656 and
5,204,055, to Sachs, et al., in which the synergistic stimulation
is a chemical binder (e.g. an adhesive), and the material is a
powder consisting of particles which bind together upon selective
application of the chemical binder.
[0007] According to a second such approach, an object is formed by
successively cutting object cross-sections having desired shapes
and sizes out of sheets of material to form object lamina.
Typically in practice, the sheets of paper are stacked and adhered
to previously cut sheets prior to their being cut, but cutting
prior to stacking and adhesion is possible. Typical of this
approach is Laminated Object Manufacturing (LOM), as described in
U.S. Pat. No. 4,752,352, to Feygin in which the material is paper,
and the means for cutting the sheets into the desired shapes and
sizes is a CO.sub.2 laser. U.S. Pat. 5,015,312 to Kinzie also
addresses building object with LOM techniques.
[0008] According to a third such approach, object laminae are
formed by selectively depositing an unsolidified, flowable material
onto a working surface in desired patterns in areas which will
become part of an object laminae. After or during selective
deposition, the selectively deposited material is solidified to
form a subsequent object lamina which is adhered to the
previously-formed and stacked object laminae. These steps are then
repeated to successively build up the object lamina-by-lamina. This
object formation technique may be generically called Selective
Deposition Modeling (SDM). The main difference between this
approach and the first approach is that the material is deposited
only in those areas which will become part of an object lamina.
Typical of this approach is Fused Deposition Modeling (FDM), as
described in U.S. Pat. Nos. 5,121,329 and 5,340,433, to Crump, in
which the material is dispensed in a flowable state into an
environment which is at a temperature below the flowable
temperature of the material, and which then hardens after being
allowed to cool. A second example is the technology described in
U.S. Pat. No. 5,260,009, to Penn. A third example is Ballistic
Particle Manufacturing (BPM), as described in U.S. Pat. Nos.
4,665,492; 5,134,569; and 5,216,616, to Masters, in which particles
are directed to specific locations to form object cross-sections. A
fourth example is Thermal Stereolithography (TSL) as described in
U.S. Pat. No. 5,141,680, to Almquist et. al.
[0009] When using SDM (as well as other RP&M building
techniques), the appropriateness of various methods and apparatus
for production of useful objects depends on a number of factors. As
these factors cannot typically be optimized simultaneously, a
selection of an appropriate building technique and associated
method and apparatus involve trade offs depending on specific needs
and circumstances. Some factors to be considered may include 1)
equipment cost, 2) operation cost, 3) production speed, 4) object
accuracy, 5) object surface finish, 6) material properties of
formed objects, 7) anticipated use of objects, 8) availability of
secondary processes for obtaining different material properties, 9)
ease of use and operator constraints, 10) required or desired
operation environment, 11) safety, and 12) post processing time and
effort.
[0010] In this regard there has been a long existing need to
simultaneously optimize as many of these parameters as possible to
more effectively build three-dimensional objects. As a first
example, there has been a need to enhance object production speed
when building objects using the third approach, SDM, as described
above (e.g. Thermal Stereolithography) while simultaneously
maintaining or reducing the equipment cost. As a second example,
there has been a long existing need for a low cost RP&M system
useable in an office environment.
[0011] In SDM, as well as the other RP&M approaches, typically
accurate formation and placement of working surfaces are required
so that outward facing cross-sectional regions can be accurately
formed and placed. The first two approaches naturally supply
working surfaces on which subsequent layers of material can be
placed and lamina formed. However, since the third approach, SDM,
does not necessarily supply a working surface, it suffers from a
particularly acute problem of accurately formed and placing
subsequent lamina which contain regions not fully supported by
previously dispensed material such as regions including outward
facing surfaces of the object in the direction of the previously
dispensed material. In the typical building process where
subsequent laminae are placed above previously formed laminae this
is particularly a problem for down-facing surfaces (down-facing
portions of laminae) of the object. This can be understood by
considering that the third approach theoretically only deposits
material in those areas of the working surface which will become
part of the corresponding object lamina. Thus, nothing will be
available to provide a working surface for or to support any
down-facing surfaces appearing on a subsequent cross-section.
Downward facing regions, as well as upward facing and continuing
cross-sectional regions, as related to photo-based
Stereolithography, but as applicable to other RP&M technologies
including SDM, are described in detail in U.S. Pat. Nos. 5,345,391,
and 5,321,622, to Hull et. al. and Snead et. al., respectively. The
previous lamina is non-existent in down-facing regions and is thus
unavailable to perform the desired support function. Similarly,
unsolidified material is not available to perform the support
function since, by definition, in the third approach, such material
is typically not deposited in areas which do not become part of an
object cross-section. The problem resulting from this situation may
be referred to as the "lack of working surface" problem.
[0012] The "lack of working surface" problem is illustrated in FIG.
1, which depicts two laminae, identified with numerals 1 and 2,
built using a three-dimensional modeling method and apparatus. As
shown, lamina 1, which is situated on top of lamina 2, has two
down-facing surfaces, which are shown with cross-hatch and
identified with numerals 3 and 4. Employing the SDM approach
described above, unsolidified material is never deposited in the
volumes directly below the down-facing surfaces, which are
identified with numerals 5 and 6. Thus, with the SDM approach,
there is nothing to provide a working surface for or to support the
two down-facing surfaces.
[0013] Several mechanisms have been proposed to address this
problem, but heretofore, none has proven completely satisfactory.
One such mechanism, suggested or described in U.S. Pat. No.
4,247,508, to Housholder; U.S. Pat. Nos. 4,961,154; 5,031,120;
5,263,130; and 5,386,500, to Pomerantz, et al.; U.S. Pat. No.
5,136,515, to Helinski; U.S. Pat. No. 5,141,680, to Almquist, et
al.; U.S. Pat. No. 5,260,009, to Penn; U.S. Pat. No. 5,287,435, to
Cohen, et al.; U.S. Pat. No. 5,362,427, to Mitchell; U.S. Pat. No.
5,398,193, to Dunghills; U.S. Pat. Nos. 5,286,573 and 5,301,415, to
Prinz, et al., involves filling the volumes below down-facing
surfaces with a support material different from that used to build
the object, and presumably easily separable from it (by means of
having a lower melting point, for example). In relation to FIG. 1,
for example, the volumes identified with numerals 5 and 6 would be
filled with the support material prior to the time that the
material used to form down-facing surfaces 3 and 4 is
deposited.
[0014] A problem with the two material (i.e. building material and
different support material) approach is that it is expensive and
cumbersome because of the inefficiencies, heat dissipation
requirements, and costs associated with handling and delivering the
support, or second, material. For example, a separate material
handling and dispensing mechanism for the support material may have
to be provided. Alternatively, means may have to be provided to
coordinate the handling and delivery of both materials through a
single system.
[0015] Another approach, described in U.S. Pat. No. 4,999,143, to
Hull, et al.; U.S. Pat. No. 5,216,616, to Masters; and U.S. Pat.
No. 5,386,500, to Pomerantz, et al., is to build generally spaced
support structures from the same material as that used to build the
object. A multitude of problems have occurred with this approach. A
first problem has involved the inability to make support structures
of arbitrary height while simultaneously ensuring that they were
easily separately from the object. Second, a problem has been
encountered regarding the inability to achieve easy separability
between object and support structure while simultaneously
maintaining an effective working surface for the building of and
support of the outward facing surfaces. A third problem involves
the inability to accumulate support structure in the direction
perpendicular to the planes of the cross-sections (e.g. vertical
direction) at approximately the same rate as that at which the
object accumulates. A fourth problem has involved the inability to
ensure easy separability and minimal damage to up-facing surfaces
when supports must be placed thereon in order to support
down-facing surfaces thereabove which are part of subsequent
layers. A fifth issue has involved the desire to increase system
throughput.
[0016] To illustrate, the objective of achieving easy separability
dictates that the surface area over which each support contacts the
object be kept as small as possible. On the other hand, the
objective of accumulating a support in the Z-direction at a rate
approximating that of object accumulation dictates that the
cross-sectional area of each support be as large as possible to
provide a large area to perimeter ratio thereby minimizing loss of
material for build up in the Z-direction due to run off, spreading,
mis-targeting and the like by allowing a large target area to
compensate for any inaccuracies in the deposition process and to
limit the ability of material to spread horizontally instead of
building up vertically.
[0017] Further, the objective of achieving minimal damage to the
down-facing surface dictates that the spacing between the supports
be kept as large as possible in order to minimize the area of
contact between the supports and the object. On the other hand, the
objective of providing an effective working surface for the
building of the down-facing surface dictates that the spacing be
kept as small as possible. As is apparent, there is a conflict in
simultaneously achieving these objectives.
[0018] This problem is illustrated in FIG. 2, in which, compared to
FIG. 1, like elements are referenced with like numerals. As shown,
down-facing surface 3 is supported through columnar supports 7a,
7b, and 7c, while down-facing surface 4 is supported through
columnar supports 8a, 8b, 8c, and 8d. Columnar supports 7a, 7b, and
7c are widely spaced from one another in order to minimize damage
to down-facing surface 3. Moreover, they are each configured to
contact the down-facing surface over a relatively small surface
area to enhance separability. On the other hand, because of their
small cross-sectional surface area, they may not be able to
accumulate, in the vertical direction, fast enough to keep up with
the rate of growth of the object. Moreover, because of their wide
spacing, they may not be able to provide an effective working
surface for the building of and support of down-facing surface
3.
[0019] Columnar supports 8a, 8b, 8c, and 8d, by contrast, are more
closely spaced together in order to provide a more effective
working surface for the building and support of down-facing surface
4. Also, each is configured with a larger surface area to enable
them to grow at rate approximating that of the object.
Unfortunately, because of their closer spacing and larger
cross-sectional area, these supports will cause more damage to the
down-facing surface when they are removed.
[0020] All patents referred to herein above in this section of the
specification are hereby incorporated by reference as if set forth
in full.
[0021] 3. Attached Appendices and Related Patents and
Applications
[0022] Appendix A is attached hereto and provides details of
preferred Thermal Stereolithography materials for use in the some
preferred embodiments of the invention.
[0023] The following applications are hereby incorporated herein by
reference as if set forth in full herein:
1 Filing Date Application No. Title Status 9/27/95 08/534,813
Selective Deposition Abandoned Modeling Method and Apparatus for
Forming Three-dimensional Objects and Supports 9/27/95 08/534,447
Method and Apparatus Abandoned for Data Manipulation and System
Control in a Selective Deposition Modeling System 9/27/95 08/53
5,772 Selective Deposition Abandoned Modeling Materials and Method
9/27/95 08/5 34,477 Selective Deposition Abandoned Modeling Method
and System
[0024] The assignee of the subject application, 3D Systems, Inc.,
is filing this application concurrently with the following related
application, which is incorporated by reference herein as though
set forth in full:
2 Application Docket No. Filing Date No. Title Status 08/722,326
9/27/96 08/722,326 Method and 5,943,235 Apparatus for Data
Manipulation and System Control in a Selective Deposition Modeling
System
[0025] According to Thermal Stereolithography and some Fused
Deposition Modeling techniques, a three-dimensional object is built
up layer by layer from a material which is heated until it is
flowable, and which is then dispensed with a dispenser. The
material may be dispensed as a semi-continuous flow of material
from the dispenser or it may alternatively be dispensed as
individual droplets. In the case where the material is dispensed as
a semi-continuous flow, it is conceivable that less stringent
working surface criteria may be acceptable. An early embodiment of
Thermal Stereolithography is described in U.S. Pat. No. 5,141,680.
Thermal Stereolithography is particularly suitable for use in an
office environment because of its ability to use non-reactive,
non-toxic materials. Moreover, the process of forming objects using
these materials need not involve the use of radiations (e.g. UV
radiation, IR radiation, visible light and/or other forms of laser
radiation), heating materials to combustible temperatures (e.g.
burning the material along cross-section boundaries as in some LOM
techniques), reactive chemicals (e.g. monomers, photopolymers) or
toxic chemicals (e.g. solvents), complicated cutting machinery, and
the like, which can be noisy or pose a significant risks if
mishandled. Instead, object formation is achieved by heating the
material to a flowable temperature then selectively dispensing the
material and allowing it to cool.
[0026] U.S. patent application Ser. No. 08/534,447, now abandoned,
referenced above, is directed to data transformation techniques for
use in converting 3D object data into support and object data for
use in a preferred Selective Deposition Modeling (SDM) system based
on SDM/TSL principles. This referenced application is also directed
to various data handling, data control, and system control
techniques for controlling the preferred SDM/TSL system described
hereinafter. Some alternative data manipulation techniques and
control techniques are also described for use in SDM systems as
well as for use in other RP&M systems.
[0027] U.S. patent application Ser. No. 08/535,772, now abandoned,
as referenced above, is directed to the preferred material used by
the preferred SDM/TSL system described herein. Some alternative
materials and methods are also described.
[0028] U.S. patent application Ser. No. 08/534,477, now abandoned,
as referenced above, is directed to some particulars of the
preferred SDM/TSL system. Some alternative configurations are also
addressed.
[0029] The assignee of the instant application, 3D Systems, Inc.,
is also the owner of a number of other U.S. Patent Applications and
U.S. Patents in RP&M field and particularly in the photo-based
Stereolithography portion of that field. These patents include
disclosures which can be combined with the teachings of the instant
application to provide enhanced SDM object formation techniques.
The following commonly owned U.S. Patent Applications and U.S.
Patents are hereby incorporated by reference as if set forth in
full herein:
3 App No. Status and/or Filing Date Topic Patent No. 08/484,582
Fundamental elements of 5,573,722 Jun 7, 1995 Stereolithography are
taught. 08/475,715 Various recoating techniques for use in
5,667,820 Jun 7, 1995 SL are described including a material
dispenser that allows for selective deposition from a plurality of
orifices. 08/479,875 Various LOM type building techniques are
5,637,169 Jun 7, 1995 described. 08/486,098 A description of curl
distortion is Abandoned Jun 7, 1995 provided along with various
techniques for reducing this distortion. 08/475,730 A description
of a 3D data slicing 5,854,748 Jun 7, 1995 technique for obtaining
cross-sectional data is described which utilizes Boolean layer
comparisons to define down-facing, up-facing and continuing
regions. Techniques for performing cure-width compensation and for
producing various object configurations relative to an initial CAD
design are also described. 08/480,670 A description of an early SL
Slicing 5,870,307 Jun 7, 1995 technique is described including
vector generation and cure width compensation. 08/428,950 Various
building techniques for use in Abandoned Apr 25, 1995 Sb are
described including various build styles involving alternate
sequencing, vector interlacing and vector offsetting for forming
semi-solid and solid objects. 08/428,951 Simultaneously Multiple
Layer Curing 5,999,184 Apr 25, 1995 techniques for Sb are taught
including techniques for performing vertical comparisons,
correcting errors due to over curing in the z-direction, techniques
for performing horizontal comparisons, and horizontal erosion
routines. 08/405,812 SL recoating techniques using 5,688,464 Mar
16, 1995 vibrational energy are described. 08/402,553 SL recoating
techniques using a 5,651,934 Mar 13, 1995 doctor blade and liquid
level control techniques are described. 08/382,268 Several SL
recoating techniques Abandoned Feb 1, 1995 are described including
techniques involving the use of ink jets to selectively dispense
material for forming a next layer of unsolidified material.
08/148,544 Fundamental elements of thermal 5,501,824 Nov 8, 1993
stereolithography are described. 07/182,801 Support structures for
SL are described. 4,999,143 Apr 18, 1988 07/183,015 Placement of
holes in objects for 5,015,424 Apr 18, 1988 reducing stress are
described. 07/365,444 Integrated SL building, cleaning and
5,143,663 Jun 12, 1989 post curing techniques are described.
07/824,819 Various aspects of a large SL 5,182,715 Jan 22, 1992
apparatus are described. 07/605,979 Techniques for enhancing
surface 5,209,878 Oct 30, 1990 finish of SL objects are described
including the use of thin fill layers in combination with thicker
structural layers and meniscus smoothing. 07/929,463 Powder coating
techniques are described 5,234,636 Aug 13, 1991 for enhancing
surface finish. 07/939,549 Building techniques for reducing curl
5,238,639 Aug 31, 1992 distortion in SL (by balancing regions of
stress and shrinkage) are described.
SUMMARY OF THE INVENTION
[0030] The instant invention embodies a number of techniques
(methods and apparatus) that can be used alone or in combination to
address a number of problems associated with building and
supporting objects formed using Selective Deposition Modeling
techniques. Though primarily directed to SDM techniques, the
techniques described hereinafter can be applied in a variety of
ways (as will be apparent to one of skill in the art who reads the
instant disclosure) to the other RP&M technologies as described
above to enhance object accuracy, surface finish, build time and/or
post processing effort and time. Furthermore, the techniques
described herein can be applied to Selective Deposition Modeling
systems that use one or more building and/or support materials
wherein one or more are selectively dispensed and in which others
may be dispensed non-selectively and wherein elevated temperatures
may or may not be used for all or part of the materials to aid in
their deposition.
[0031] The techniques can be applied to SDM systems wherein the
building material (e.g. paint or ink) is made flowable for
dispensing purposes by adding a solvent (e.g. water, alcohol,
acetone, paint thinner, or other solvents appropriate for specific
building, wherein the material is solidifiable after or during
dispensing by causing the removal of the solvent (e.g. by heating
the dispensed material, by dispensing the material into a partially
evacuated (i.e. vacuumed) building chamber, or by simply allowing
sufficient time for the solvent to evaporate). Alternatively, or
additionally, the building material (e.g. paint) may be thixotropic
in nature wherein an increase in shear force on the material could
be used to aid in its dispensing or the thixotropic property may
simply be used to aid the material in holding its shape after being
dispensed. Alternatively, and/or additionally, the material may be
reactive in nature (e.g. a photopolymer, thermal polymer, one or
two-part epoxy material, a combination material such as one of the
mentioned materials in combination with a wax or thermal plastic
material) or at least solidifiable when combined with another
material (e.g. plaster of paris & water), wherein after
dispensing, the material is reacted by appropriate application of
prescribed stimulation (e.g. heat, EM radiation [visible, IR, UV,
x-rays, etc.], a reactive chemical, the second part of a two part
epoxy, the second or multiple part of a combination) such that the
building material and/or combination of materials become
solidified. Of course, Thermal Stereolithographic materials and
dispensing techniques may be used alone or in combination with the
above alternatives. Furthermore, various dispensing techniques may
be used such as dispensing by single or multiple ink jet devices
including, but not limited to, hot melt ink jets, bubble jets,
etc., and continuous or semi-continuous flow, single or multiple
orifice extrusion nozzles or heads.
[0032] Accordingly it is a first object of the invention to provide
a method and apparatus for higher accuracy production of
objects.
[0033] A second object of the invention is to provide a method and
apparatus for production of objects with less distortion by
controlling the thermal environment during object formation.
[0034] A third object of the invention is to provide a method and
apparatus for production of objects with less distortion by
controlling how material is dispensed.
[0035] A fourth object of the invention is to provide a method and
apparatus for enhancing object production speed.
[0036] A fifth object of the invention is to provide a support
structure method and apparatus that allows object supports of
arbitrary height to be formed.
[0037] A sixth object of the invention is to provide a support
structure method and apparatus that provides a good working
surface.
[0038] A seventh object of the invention is to provide a method and
apparatus that forms a support structure that is easily removed
from down-facing surfaces of the object.
[0039] An eighth object of the invention is to provide a support
structure method and apparatus that results in minimum damage to
down-facing surfaces of the object upon removal thereof.
[0040] A ninth object of the invention is to provide a method and
apparatus for removing the supports from the object.
[0041] A tenth object of the invention is to provide a support
structure method and apparatus that builds up supports vertically
at a rate approximating the vertical build up rate of the
object.
[0042] An eleventh object of the invention is to provide a method
and apparatus that forms a support structure that is easily removed
from up-facing surfaces of the object.
[0043] A twelfth object of the invention is to provide a support
structure method and apparatus that results in minimum damage to
up-facing surfaces of the object upon removal thereof.
[0044] A thirteenth object of the invention is to provide a method
and apparatus for producing supports that are separated from
vertical object surfaces.
[0045] A fourteenth object is to provide support structures that
are combinable with other RP&M technologies for enhanced object
formation.
[0046] It is intended that the above objects can be achieved
separately by different aspects of the invention and that
additional objects of the invention will involve various
combinations of the above independent objects such that combined
benefits may be obtained from combined techniques.
[0047] Other objects of the invention will be apparent from the
description herein.
BRIEF DESCRIPTION OF THE DRAWINGS
[0048] FIG. 1 illustrates down-facing surfaces of an object;
[0049] FIG. 2 illustrates two categories of support structures for
supporting the down-facing surfaces of FIG. 1;
[0050] FIG. 3 is a diagram of the main functional components of the
preferred Selective Deposition Model/Thermal Stereolithography
system;
[0051] FIGS. 4a and 4b illustrate the orifice plate of the print
head of FIG. 3 at different orientations to the scanning
direction;
[0052] FIG. 5 is a more detailed drawing of the planarizer of FIG.
3;
[0053] FIG. 6 illustrates the relative spacing between adjacent
nozzles on the orifice plate and adjacent raster lines;
[0054] FIG. 7 illustrates the grid of pixels which defines the data
resolution of the system;
[0055] FIG. 8 illustrates two perpendicular examples of raster line
orientation;
[0056] FIG. 9 illustrates two examples of deposition propagation in
the secondary scanning direction;
[0057] FIGS. 10a and 10b illustrate two examples of deposition
propagation in the main scanning direction;
[0058] FIGS. 11a and 11b illustrate an example of scan line
interlacing;
[0059] FIGS. 12a and 12b illustrate an example of drop location
interlacing along several scan lines;
[0060] FIGS. 13a and 13b illustrate a further example of drop
location interlacing along several scan lines;
[0061] FIG. 14 illustrates a single pixel checkerboard deposition
pattern;
[0062] FIG. 15 illustrates a 3.times.3 column support pixel pattern
forming a preferred support structure;
[0063] FIGS. 16a-16d illustrates several overprinting schemes;
[0064] FIGS. 17a and 17b illustrates a mis-registration problem
that can occur when using an overprinting technique;
[0065] FIG. 18 illustrates the resulting deposition regions when
the pixels of FIG. 15 are exposed using an overprinting scheme;
[0066] FIG. 19 illustrates an alternative pixel pattern for column
supports;
[0067] FIG. 20 illustrates a Hybrid support structure;
[0068] FIGS. 21a and 21b illustrate arch-type supports;
[0069] FIGS. 22a-d depict an interlacing embodiment for depositing
material during the building of an object;
[0070] FIGS. 23a-h illustrate a building embodiment which uses
horizontal and vertical pixel offsets;
[0071] FIGS. 24a-d illustrate a deposition embodiment that reduces
risk of bridging between regions separated by a gap;
[0072] FIGS. 25a-e illustrate a building technique wherein the
object is separated into pieces, built separately and then adhered
together;
[0073] FIG. 26 illustrates a preferred two step raster scanning and
indexing pattern;
[0074] FIGS. 27a-27e depict various combinations of working surface
and targeting positions;
[0075] FIG. 28a depicts a side view of an embodiment of branching
supports;
[0076] FIG. 28b depicts a side view of another embodiment of
branching supports;
[0077] FIGS. 29a-29e depict a top view of branching layers for an
embodiment of branching supports;
[0078] FIGS. 30a-30m depict a top view of branching layers for
another embodiment of branching supports;
[0079] FIGS. 31a-31c depict a top view of branching layers for
another embodiment of branching supports; and
[0080] FIGS. 32a-31d depict a top view of branching layers for
another embodiment of branching supports
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0081] As previously discussed, the subject application is directed
to support techniques and building techniques appropriate for use
in a Selective Deposition Modeling (SDM) system. In particular, the
preferred SDM system is a Thermal Stereolithography (TSL) system.
The Description Of The Preferred Embodiments will begin with a
description of the preferred TSL system. A more detailed
description of the preferred system, data manipulation techniques,
system control techniques, material formulations and properties,
and various alternatives are described in previously referenced and
incorporated U.S. patent application Ser. Nos. 08/534,813;
08/534,447, now abandoned; 08/535,772; and 08/534,477, now
abandoned; and U.S. Pat. No. 5,943,235, filed concurrently
herewith. Further, alternative systems are discussed in a number of
the previously incorporated applications and patents, especially
those referenced as being directly related to, or applicable to,
SDM, TSL or Fused Deposition Modeling (FDM). As such, the support
structures and build styles described hereinafter should be
construed as applicable to a variety of SDM, TSL and FDM systems
and not limited by the system examples described herein.
Furthermore, as noted previously, these support structures and
build styles have utility in the other RP&M technologies.
[0082] A preferred embodiment of an apparatus for performing
SDM/TSL is illustrated in FIG. 3. The apparatus comprises a
dispensing platform 18, a dispensing head 9 (e.g. multi orifice ink
jet head), wherein the dispensing head 9 is located on the
dispensing platform 18, a planarizer 11 and a part-building
platform, 15. The dispensing platform 18 is a horizontal member
which is capable of supporting the planarizer 11 and the dispensing
head 9. The dispensing platform 18 is slidably coupled to an
X-stage 12 through a coupling member 13. The X-stage 12 is
preferably controlled by a control computer or microprocessor (not
shown) and controllably moves the dispensing platform 18 back and
forth in the X-direction, or the main scanning direction.
[0083] Furthermore, at either side of the platform 18, fans (not
shown) are mounted for blowing air vertically downward to help cool
the dispensed material 14 and part-building platform 15 such that
the desired building temperature is maintained. Other suitable
mounting schemes for the fans and/or other cooling systems include,
but are not limited to, misting devices for directing vaporizable
liquids (e.g. water, alcohol, or solvents) onto the surface of the
object, forced air cooling devices with fans mounted between the
planarizer 11 and the dispensing head 9, and forced air cooling
devices with stationary or moving fans mounted off the dispensing
platform. Cooling systems may include active or passive techniques
for removing heat which may be computer controlled in combination
with temperature sensing devices to maintain the previously
dispensed material within the desired building temperature range.
Other approaches to cooling include, but are not limited to,
salting the material with a substance which functions as a black
body radiator, especially at IR frequencies, such that heat is more
readily radiated from the object during the building process.
Further alternative approaches include, but are not limited to,
adding a conductive substance to the material every few layers,
adding a solvent to the material, building parts with cooling
passages or with an embedded substrate (such as interlaced wires)
for cooling, or building on a glass plate or Mylar.RTM. sheet.
[0084] Other embodiments for cooling the material or at least
maintaining the dispensed material at an appropriate temperature,
might involve the use of directing a temperature moderating gas
(e.g. a cooling gas such as air) at the upper surface of the
partially formed object, as discussed above, but they may
additionally include controlled techniques for removing the cooling
air from the surface. Such techniques might involve the use of
blowing and sucking devices and alternate positioning of blowing
ducts (gas inserting ducts) and sucking ducts (gas removing ducts).
These ducts may allow the cooling gas to be removed before
excessive heating of the gas causes a loss in effective cooling
rate. The gas directed at the surface may be introduced in a cooled
state, introduced at room temperature, or introduced at some other
appropriate temperature. If appropriately configured, these
alternate inserting and removing ducts may allow faster scanning
speed than presently allowable due to turbulence or wind distortion
of fragile structures such as supports. These ducts might be
configured to provide air flow in the opposite direction to print
head movement thereby reducing the net wind velocity coming into
contact with the partially formed object. The blowing or sucking
associated with individual ducts may be reversed, turned on, or
turned off depending on the direction of movement of the print
head.
[0085] The print head 9 is a commercial print head configured for
jetting hot melt inks such as, for example, thermal plastics or
wax-like materials, and modified for use in a three-dimensional
modeling system, wherein the print head undergoes back and forth
movements and accelerations. The print head modifications include
configuring any on board reservoir such that the accelerations
result in minimal misplacement of material in the reservoir. One
preferred embodiment includes a 96 jet commercial print head, Model
No. HDS 96i, sold by Spectra Corporation, Nashua, Hew Hampshire
including reservoir modifications. The print head is supplied
material in a flowable state from a Material Packaging &
Handling Subsystem (not shown), which is described in the
previously referenced U.S. patent application Ser. No. 08/534,477,
now abandoned. In the preferred embodiment, all 96 jets on the head
are computer controlled to selectively fire droplets through
orifice plate 10 when each orifice (i.e. jet) is appropriately
located to dispense droplets onto desired locations. In practice,
approximately 12,000 to 16,000 commands per second have been sent
to each jet selectively commanding each one to fire (dispense a
droplet) or not to fire (not to dispense a droplet) depending on
jet position and desired locations for material deposition. Also,
in practice, firing commands have been sent simultaneously to all
jets. Since, the preferred print head mentioned above contains
almost 100 jets, the above noted firing rates result in the need to
send approximately 1.2 to 1.6 million firing commands to the head
each second. Thus, the head is computer controlled so as to
selectively fire the jets and cause them to simultaneously emit
droplets of the molten material through one or more orifices in
orifice plate 10. Of course, it will be appreciated that in
alternative preferred embodiments, heads with different numbers of
jets can be used, different firing frequencies are possible, and in
appropriate circumstances non-simultaneous firing of the jets is
possible.
[0086] To most effectively build a three-dimensional object, it is
desired that all of the jets fire correctly. To ensure that all
jets are firing correctly or at least maximize the number which are
firing correctly, various techniques can be used. One such
embodiment involves checking the jets after formation of each
lamina. This technique includes the steps of: 1) forming a lamina;
2) checking the jets by printing a test pattern of lines on a piece
of paper, with all jets firing; 3) optically detecting (through bar
code scanning or the like) whether a jet is misfiring; 4)
unclogging the jet; 5) removing the entirety of the just-dispensed
layer (e.g. by machining using a preferred planarizer to be
described herein after); and 6) rebuilding the layer with all jets
including the unclogged jet.
[0087] A second embodiment involves the following preferred steps:
1) forming a layer; 2) optically detecting a misfiring jet; 3)
rescanning the lines on the layer that should have been formed by
the misfiring jet; 4) ceasing the use of the misfiring jet in the
remainder of the building process; and 5) scanning subsequent
layers while compensating for the misfiring jet (i.e., make extra
passes with a working jet to cover the lines corresponding to the
misfiring jet). Optionally, the misfiring jet may be periodically
checked to see if it has started functioning again. If so, this jet
is put back into operation. Another option involves putting a
misfiring jet through a reactivation routine to see if it can be
made operational. This could occur during the building process or
during servicing of the system. As a further alternative, it may be
possible to determine whether or not a jet is firing correctly by
tracking the electrical characteristics of the piezo electric
element as firing is to occur.
[0088] A third embodiment might involve the use of a flexible
element for wiping excess material from the bottom of the print
head. This embodiment involves the firing of all the jets followed
by a wiping of the orifice plate with a heated rubber (e.g. VITON)
blade. Preferably the blade is positioned such that it contacts the
orifice plate as they are relatively moved passed each other
thereby causing a squeegee action to remove excess material from
the orifice plate and hopefully revitalizing any jets which were
not behaving properly. It is further preferred that the orifice
plate and blade be positioned at an angle to each other such that
at any one time during their contact only a portion of the orifice
plate is in contact with the squeegee thereby minimizing the force
the blade exerts on the orifice plate.
[0089] The orifice plate 10 is mounted on the dispensing platform
18 such that droplets of material are allowed to emit from the
underside of the dispensing platform 18. The orifice plate 10 is
illustrated in FIGS. 4a and 4b. In one preferred embodiment, and as
depicted in FIG. 4a, the orifice plate 10 (i.e. the line of
orifices) is mounted approximately perpendicular to the main
scanning direction (X-direction) and is configured with N=96
individually controllable orifices (labeled 10(1), 10(2), 10(3) . .
. 10(96)). Each orifice is equipped with a piezoelectric element
that causes a pressure wave to propagate through the material when
an electric firing pulse is applied to the element. The pressure
wave causes a drop of material to be emitted from the orifice. The
96 orifices are controlled by the control computer which controls
the rate and timing of the firing pulses applied to the individual
orifices. With reference to FIG. 4a, the distance "d" between
adjacent orifices in the preferred embodiment is about {fraction
(8/300)} of an inch (about 26.67 mils or 0.677 mm). Thus, with 96
orifices, the effective length "D" of the orifice plate is about
(N.times.{fraction (8/300)} inch)=(96.times.{fraction (8/300)}
inches)=2.56 inches (65.02 mm).
[0090] To accurately build an object, the print head must fire such
that the droplets reach particular "desired drop locations", i.e.,
locations that the droplet is intended to land. The desired drop
locations are determined from a data map, or pixel locations, which
describes the object as a series of relatively spaced location
points. For the droplets to land at the desired drop locations, the
print head must fire the droplets from a "desired firing location"
or at a "desired firing time" which is based on the relative
position of the print head to the desired drop location, the
velocity of the print head, and the ballistic characteristics of
the particles after being ejected.
[0091] In a preferred embodiment raster scanning is used to
position the print head 9 and orifices at desired firing locations.
The printing process for each lamina is accomplished by a series of
relative movements between the head 9 and the desired drop or
firing locations. Printing typically occurs as the head 9
relatively moves in a main scanning direction. This is followed by
a typically smaller increment of movement in a secondary scanning
direction while no dispensing occurs, which in turn is followed by
a reverse scan in the main scanning direction in which dispensing
again occurs. The process of alternating main scans and secondary
scans occurs repeatedly until the lamina is completely
deposited.
[0092] Alternative preferred embodiments may perform small
secondary scanning movements while main scanning occurs. Because of
the typically large difference in net scanning speed along the main
and secondary directions such an alternative still results in
deposition along scanning lines which are nearly parallel to the
main scanning direction and perpendicular to the secondary scanning
direction. Further alternative preferred embodiments may utilize
vector scanning techniques or a combination of vector scanning and
raster scanning techniques.
[0093] It has been found that droplets, immediately after being
dispensed from the jet orifice, have an elongated shape, compared
to their width. The ratio of droplet length to width can be defined
as the aspect ratio of the droplet. It has further been found that
the aspect ratio of these droplets becomes smaller as the droplets
travel away from the jet orifice (i.e. they become more spherical
in shape).
[0094] It should be appreciated that in some embodiments the
spacing between the orifice plate 10 and the working surface is
preferably large enough such that the drops emitted therefrom have
become semi-circular in shape when they impact the working surface.
On the other hand, it should also be appreciated that this spacing,
which determines the distance the droplets must travel during the
printing process before impact, should be minimized in order to
avoid accuracy problems which may occur as the travel time is
increased. In practice, it has been found that these two conditions
are both satisfactorily met when at least 90% of the droplets
emitted from the orifice plate have achieved an aspect ratio (i.e.,
the ratio formed by the width of the droplet divided by its length)
which is preferably less than about 1.3, more preferably less than
about 1.2, and most preferably, which is between about 1.05 and
1.1.
[0095] In alternative preferred embodiments, the print head 9 may
be mounted at a non-perpendicular angle to the main scanning
direction. This situation is depicted in FIG. 4b wherein the print
head 9 is mounted at an angle ".alpha." to the main scanning
direction (e.g., the "X" direction). In this alternative situation
the separation between the orifices is reduced from d to
d'=(d.times.sin .alpha.) and the effective length of the print head
9 is reduced to D'=(D.times.sin .alpha.). When the spacing d' is
equal to the desired print resolution in the secondary scanning
direction (direction approximately perpendicular to the main
scanning direction), the angle .alpha. is considered to be the
"saber angle".
[0096] If the spacing d or d' is not at the desired secondary print
resolution (i.e. the print head is not at the saber angle) then for
optimal efficiency in printing a layer, the desired resolution must
be selected such as to make d or d' an integer multiple of the
desired resolution. Similarly, when printing with
.alpha..noteq.90.degree., a spacing between jets exists in the main
scanning direction as well as the secondary scanning direction.
This spacing is defined by d"=d.times.cos .alpha.. This in turn
dictates that optimization of printing efficiency will occur when
the desired main direction print resolution is selected to be an
integral divisor of d" (this assumes that firing locations are
located in a rectangular grid). Another way of expressing this is
that the angle .alpha. is selected such that d' and/or d" when
divided by appropriate integers M and P yield the desired secondary
and main scanning resolutions. An advantage to using the preferred
print head orientation (.alpha.=90.degree.) is that it allows any
desired printing resolution in the main scanning direction while
still maintaining optimal efficiency.
[0097] In other preferred embodiments, multiple heads may be used
which lay end to end (extend in the secondary scanning direction)
and/or which are stacked back to back (stacked in the main scanning
direction). When stacked back to back, the print heads may have
orifices aligned in the main scanning direction so that they print
over the same lines or alternatively they may be offset from one
another so as to dispense material along different main scanning
lines. In particular, it may be desirable to have the back to back
print heads offset from each other in the secondary scanning
direction by the desired raster line spacing to minimize the number
of main scanning passes that must occur. In other preferred
embodiments, the data defining deposition locations may not be
located by pixels configured in a rectangular grid but instead may
be located by pixels configured in some other pattern (e.g. offset
or staggered pattern). More particularly, the deposition locations
may be fully or partially varied from layer to layer in order to
perform partial pixel drop location offsetting for an entire layer
or for a portion of a layer based on the particulars of a region to
be jetted.
[0098] Presently preferred printing techniques involve deposition
resolutions of 300, 600 and 1200 drops per inch in the main
scanning direction and 300 drops per inch in the secondary scanning
direction.
[0099] With reference to FIGS. 3 and 5, the planarizer 11 includes
a heated rotating (e.g. 2000 rpm) cylinder 18a with a textured
(e.g. knurled) surface. Its function is to melt, transfer and
remove portions of the previously dispensed layer or lamina of
material in order to smooth it out, to set a desired thickness for
the last formed layer, and to set the net upper surface of the last
formed layer to a desired level. Numeral 19 identifies a layer of
material which was just deposited by the print head. The rotating
cylinder 18a is mounted in the dispensing platform such that it is
allowed to project from the underside of the platform by a
sufficient amount in the Z-direction such that it contacts material
19 at a desired level. More importantly the rotating cylinder 18a
is mounted so as to project a desired distance below the plane
swept out by the underside of the print head or orifice plate. In
the event that the orifice plate itself projects below the
dispensing platform 18, the rotating cylinder 18a will project
further below the dispensing platform 18. In one preferred
embodiment, the projection below the orifice plate in the
z-direction is in the range of 0.5 mm to 1.0 mm. The extent to
which the roller extends below the dispensing platform 18 is a
determinant of the spacing between the orifice plate 10 and the
working surface. Thus, in some preferred embodiments it is
preferred that the extent to which the planarizer 11 extends below
the orifice plate 10 not conflict with the condition described
earlier in relation to droplet aspect ratio, in which 90% of the
droplets have achieved an aspect ratio upon impact preferably less
than about 1.3, more preferably less than about 1.2, and most
preferably between about 1.05-1.1.
[0100] The rotation of the cylinder sweeps material from the
just-deposited layer, identified in the figure with numeral 21,
leaving in its wake smooth surface 20. The material 21 adheres to
the knurled surface of the cylinder and is displaced until it
contacts wiper 22. As shown, wiper 22 is disposed to effectively
"scrape" the material 21 from the surface of the cylinder. The
wiper is preferably made of VITON, although other materials, such
as TEFLON.RTM., are capable of scraping the material from the
surface of the cylinder are also suitable. Preferably the scrapper
material is non-wetting with respect to the liquefied building
material and is durable enough to contact the rotating cylinder 18a
without wearing out too quickly. The removed material is drawn away
under suction via a heated umbilical to a waste tank (not shown),
where it is either disposed of or recycled. The planarizer waste
tank is held constantly under vacuum in order to continuously
remove material from the planarizer cylinder. When the tank becomes
full the system automatically reverses the vacuum for a few seconds
to purge the waste material out of a check valve into a larger
waste tray. Once empty, vacuum is restored and waste continues to
be drawn from the planarizer. In practice, it has been observed
that approximately 10-15% of the material dispensed is removed by
the planarizer. Though most preferred embodiments use a combination
of rotating, melting and scraping to perform planarization, it is
believed that other embodiments might utilize any one of these
three elements or any combination of two of them.
[0101] In present implementations, the cylinder 18a rotates (e.g.
at approximately 2000 rpm) in a single direction as the head moves
back in forth in each direction. In alternative embodiments, the
cylinder 18a can be made to rotate in opposite directions based on
the forward or reverse direction that platform 18 sweeps while
moving in the main scanning direction. Some embodiments might
involve the axis of rotation of cylinder 18a being off axis
relative to the axis of orientation of the print head. In other
embodiments more than one cylinder 18a may be used. For example, if
two cylinders were used, each one may be caused to rotate in
different directions and may further be vertically positionable so
as to allow a selected one to participate in planarization during
any given sweep.
[0102] When using a single print head 10 and cylinder 18a,
planarization only effectively occurs on every second pass of the
print head though deposition occurs on each pass (i.e.
planarization always occurs in the same direction). Under these
conditions, planarization occurs when the sweeping direction points
along the same direction as an arrow pointing from the cylinder to
the print head. In other words planarization occurs when the
sweeping direction is such that the cylinder follows the print head
as the elements traverse the layer in the main scanning
direction.
[0103] Other preferred embodiments might use a single cylinder, but
use one or more print heads located on either side of the cylinder,
such that planarization effectively occurs when sweeping in both
directions. Other alternative embodiments might decouple the
movement of the print head(s) and the planarizing cylinder. This
decoupling might allow independent planarization and dispensing
activity. Such decoupling might involve the directions of print
head sweeping (e.g. X-direction) and cylinder sweeping (e.g.
Y-direction) being different. Such decoupling might also allow
multiple layers to be formed or lines of a single layer to be
deposited between planarization steps.
[0104] With reference to FIG. 3, part-building platform 15 is also
provided. The three-dimensional object or part, identified in the
figure with reference numeral 14, is built on the platform 15. The
platform 15 is slidably coupled to Y-stage 16a and 16b which
controllably moves the platform back and forth in the Y-direction
(i.e., index direction or secondary scanning direction) under
computer control. The platform 15 is also coupled to Z-stage 17
which controllably moves the platform up and down (typically
progressively downward during the build process) in the Z-direction
under computer control.
[0105] To build a cross-section, lamina, or layer of a part, the
Z-stage is directed to move the part-building platform 15 relative
to the print head 9 such that the last-built cross-section of the
part 14 is situated an appropriate amount below the orifice plate
10 of the print head 9. The print head 9 in combination with the
Y-stage 16a, 16b is then caused to sweep one or more times over the
XY build region (the head sweeps back and forth in the X direction,
while the Y-stage 16a, 16b translates the partially formed object
in the Y-direction). The combination of the last formed
cross-section, lamina, or layer of the object and any supports
associated therewith define the working surface for deposition of
the next lamina and any supports associated therewith. During
translation in the XY directions, the jet orifices of the print
head 9 are fired in a registered manner with respect to previously
dispensed layers to deposit material in a desired pattern and
sequence for the building of the next lamina of the object. During
the dispensing process a portion of the dispensed material is
removed by the planarizer 11 in the manner discussed above. The X,
Y and Z movements, dispensing, and planarizing are repeated to
build up the object from a plurality of selectively dispensed and
adhered layers. Moreover, platform 15 can be indexed in either the
Y- or Z- direction while the direction of the dispensing platform
18 is in the process of being reversed upon the completion of a
scan.
[0106] In a preferred embodiment, the material deposited during the
formation of a lamina has a thickness at or somewhat greater than
the desired layer thickness. As described above the excess material
deposited is removed by the action of the planarizer. Under these
conditions, the actual build up thickness between layers is not
determined by the amount of material deposited for each layer but
instead is determined by the down-ward vertical increment made by
the platform after deposition of each layer. If one wants to
optimize build speed and/or minimize the amount of material wasted,
it is desirable to trim off as little material as possible during
the deposition process. The less material trimmed off, the thicker
each lamina is and the faster the object builds up. On the other
hand if one makes the layer thickness, i.e. z-increment, too large
then the amount of build up associated with at least some drop
locations will begin to lag behind the desired level. This lag will
results in the actual physical working surface being at a different
position from the desired working surface and probably results in
the formation of a non-planar working surface. This difference in
position can result in the XY misplacement of droplets due to a
longer time of flight for than expected and it can further result
in the vertical misplacement of object features that happen to
begin or end at the layers in which the actual working surface is
mis-positioned. Therefor in some embodiments it is desirable to
optimize layer incrementing in the vertical direction.
[0107] To determine an optimum Z-axis increment, an accumulation
diagnostic part may be used. This technique preferably involves
building layers of one or more test parts at successively greater
Z-increments, measuring the height of the features formed and
determining which Z-increments gave rise to formation heights (i.e.
vertical accumulation) of the correct amount and which Z-increments
gave rise to formation heights which lagged behind the desired
amount. It is expected that layer-increments (i.e. Z-increments) up
to a certain amount (i.e. the maximum acceptable amount) would
yield build up levels for the object equal to that predicted by the
product of the number of layers and the thickness of each layer.
After the layer increment exceeds the maximum acceptable amount,
the build up level of the object would fall short of the amount
predicted by the product of the number of layers and the thickness
of each layer. Alternatively, the planarity of upper surface of the
diagnostic part(s) may be lost (indicating that some drop locations
may be receiving sufficient material while others are not). By
inspecting the diagnostic part(s), the maximum acceptable
Z-increment amount can be empirically determined. The optimal
Z-increment amount can then be selected as this maximum acceptable
amount or it can be selected at some thickness somewhat less than
this maximum amount. Since it is known that different build and
support styles accumulate in the vertical direction at different
rates, the above test can be performed for each build style and
support style, wherefrom the optimal Z-increment for a combination
of different styles can then be selected such that it is not
thicker than any of the maximum amounts determined for each style
individually.
[0108] Further, the dispensing head, in tracing a given scan line,
may only maintain a substantially constant velocity over part of
the scan line. During the remainder of the scan, the head 9 will
either be accelerating or decelerating. Depending on how the firing
of the jets is controlled this may or may not cause a problem with
excess build up during the acceleration and deceleration phases of
the motion. In the event that velocity changes can cause problem in
a accumulation rate, the part or support building can be confined
to the portion of the scan line over which the print head has a
substantially constant velocity. Alternatively, as discussed in the
concurrently filed U.S. patent application Ser. No. 08/722,326, a
firing control scheme can be used which allows accurate deposition
during the acceleration or deceleration portions of a scan
line.
[0109] As noted previously, in some preferred embodiments, the
print head 9 is directed to trace a raster pattern. An example of
this is depicted in FIG. 6. As shown, the raster pattern consists
of a series of raster lines (or scan lines), R(1), R(2), . . . ,
R(N), running in the X-direction or main scanning direction and
arrayed (i.e. spaced) along the Y-direction (i.e. the index
direction or secondary scanning direction). The raster lines are
spaced from one another by a distance d.sub.r, which, in one
preferred embodiment, is {fraction (1/300)} of an inch (about 3.3
mils or about 83.8 .mu.m). Since the orifices of the print head 9
are spaced by the distance d, which as discussed above is
preferably about 26.67 mils (0.6774 .mu.m), and since the desired
number of raster lines may extend in the index direction by a
distance greater than the length of the orifice plate 10, about
2.56 inches (65.02 mm), the print head 9 must be swept over the
working surface through multiple passes in order to trace out all
desired raster lines.
[0110] This is preferably accomplished by following a two-step
process. In the first step, the print head 9 is passed 8 times over
the working surface in the main scanning direction, with the
Y-stage 16a, 16b being indexed by the amount d.sub.r in the
secondary scanning direction after each pass in the main scanning
direction. In the second step, the Y-stage 16a, 16b is indexed by a
distance equal to the length of the orifice plate 10 (2.5600
inches+d.sub.r (0.0267 inches)=2.5867 inches (65.70 mm). This
two-step process is then repeated until all of the desired raster
lines have been traced.
[0111] In a first pass, for example, the print head 9 might be
directed to trace raster lines R(1) (via orifice 10(1) in FIG. 4),
R(9) (via orifice 10(2)), R(17) (via orifice 10(3)), etc. The
Y-stage 16a, 16b would then be directed to move the building
platform 18 the distance d.sub.r (one raster line) in the index
direction. On the next pass, the print head 9 might be directed to
trace R(2) (via 10(1)), R(10) (via 10(2)), R(17) (via 10(3)), etc.
Six more passes would then be performed with the Y-stage 16a, 16b
being indexed by the distance d.sub.r after each pass, until a
total of 8 passes have been performed.
[0112] After performing the first step (consisting of 8 passes),
the second step is performed if there are more raster lines to be
traced. The second step consists of directing the Y-stage to move
the building platform by an amount equal to the full length of the
orifice plate 10+d.sub.r, 2.5867 inches (65.70 mm). As needed,
another set of 8 passes, comprising the first step, is performed
followed by another second step. The two-step process described
above would then be repeated until all raster lines have been
traced out.
[0113] An example of this two step process is depicted in FIG. 26
for a print head consisting of two jets and wherein the two jets
are separated one from the other by 8 raster spacings. The scanning
of the cross-sections begins with the first jet located at position
201 and the second jet located at position 301. The first step of
the scanning process begins with the scanning of raster lines 211
and 311 in the indicated direction by the first and second jets,
respectively. As part of the first step the initial scanning of
raster lines 211 and 311 is followed by an index increment of one
raster line width as indicated by elements 221 and 321. Continuing
as part of the first step, the initial raster scan and index
increment are followed by seven more raster scans (depicted by
pairs of lines 212 and 312, 213 and 313, 214 and 314, 215 and 315,
216 and 316, 217 and 317, and 218 and 318) separated by six more 1
raster line width index increments (depicted with pairs of elements
222 and 322, 223 and 323, 224 and 324, 225 and 325, 226 and 326,
and 227 and 327). Immediately after scanning raster line pairs 218
and 318, the second step of the process is taken wherein the head
is indexed in the Y-direction according to the direction and
lengths of raster lines 228 and 229. The length of this index is
equal to the head width (i.e. in this example 8 raster lines
widths) plus the width of 1 more raster line. After this large
increment, the first steps and second steps are repeated as many
times as necessary to complete the scanning of the particular
cross-section being formed. It will be apparent to one of skill in
the art that this two step scanning technique can be implemented in
other ways in alternative embodiments. For example the second step
may, instead of consisting of the positive index increment in Y as
indicated by elements 228 and 328, consist of the large negative
increment in Y as indicated by element 330 (i.e. three head widths
minus one raster line width).
[0114] This preferred embodiment may be summarized as including the
following characteristics: 1) the spacing along an indexing
direction between adjacent jets is an integral (N) multiple of the
desired spacing (d.sub.r) between adjacent deposition lines which
extend in a printing direction which is approximately perpendicular
to the indexing direction; 2) the first step includes performing a
number of passes (N) in the printing direction where each pass is
offset in the indexing direction by the desired spacing (d.sub.r)
between adjacent deposition lines; and 3) the second step includes
offsetting the print head 9 in the indexing direction by a large
amount such that the jets can deposit material in material in
another N passes, wherein successive passes are separated by one
raster line index increments, and whereafter another large index
increment will be made as necessary. In most preferred embodiments
the second step index amount will be equal to the sum of the
spacing between the first jet and the last jet plus the desired
spacing between adjacent deposition lines (i.e., N.times.J+d.sub.r,
where J is the number of jets on the print head 9).
[0115] As noted in the above example, other second step index
amounts are possible. For example, negative second step increments
(opposite direction to the index increments used in the first step)
equal to the sum of the head width plus the product of two times
the width between successive jets less the width of one raster line
spacing. In other embodiments, it is possible to use second step
index amounts which vary or which alternate back and forth between
positive and negative values. In these embodiments the second step
increment amount has the common feature that it is larger than the
individual index amounts used in the first step.
[0116] In other preferred embodiments other single or multiple step
indexing patterns can be used, index direction increments could be
generally be made which include increments involving both negative
and positive movements along the Y-axis. This might be done to scan
raster lines that were initially skipped. This will be described
further in association with a technique called "interlacing".
[0117] In some preferred embodiments, the firing of ink jets is
controlled by a rectangular bit map, i.e., pixel locations,
maintained in the control computer or other memory device. The bit
map consists of a grid of memory cells, in which each memory cell
corresponds to a pixel of the working surface, and in which the
rows of the grid extend in the main scanning direction
(X-direction) and the columns of the grid extend in the secondary
scanning direction (Y-direction). The width of (or distance
between) the rows (spacing along the Y-direction) may be different
from the width (or length of or distance between) of the columns
(spacing along the X-direction) dictating that different data
resolutions may exist along the X and Y directions. In other
preferred embodiments, non-uniform pixel size is possible within a
layer or between layers wherein one or both of the pixel width or
length is varied by pixel position. In still other preferred
embodiments, other pixel alignment patterns are possible. For
example, pixels on adjacent rows may be offset in the main scanning
direction by a fractional amount of the spacing between pixels so
that their center points do not align with the center points of the
pixels in the neighboring rows. This fractional amount may be 1/2
so that their center points are aligned with the pixel boundaries
of adjacent rows. It may be 1/3, 1/4 or some other amount such that
its takes two or more intermediate layers before pixel patterns
realign on subsequent layers. In further alternatives, pixel
alignment might be dependent on the geometry of the object or
support structure being dispensed. For example, it might be
desirable to shift pixel alignment when forming a portion of a
support pattern that is supposed to bridge a gap between support
columns or when forming a down-facing portion of an object. These
and other alternative pixel alignment schemes can be implemented by
modifying the pixel configuration, or alternatively, defining a
higher resolution pixel arrangement (in X and/or Y) and using pixel
firing patterns that do not fire on every pixel location but
instead fire on selected spaced pixel locations which may vary
according to a desired random, predetermined or object biased
pattern.
[0118] The data resolution in the main scanning direction can be
defined in terms of Main Direction Pixels (MDPs). MDPs may be
described in terms of pixel length or in terms of number of pixels
per unit length. In some preferred embodiments, MDP=300 pixels per
inch (26.67 mils/pixel or 677.4 .mu.m/pixel). In other preferred
embodiments, MDP=1200 pixels per inch. Of course any other MDP
values can be used as desired. Similarly the data resolution in the
secondary scanning direction may be defined in terms of Secondary
Direction Pixels (SDPs) and the SDPs may be described in terms of
pixel width or in terms of number of pixels per unit length. In
some preferred embodiments SDP=MDP=300 pixels per inch (26.67
mils/pixel or 677.4 .mu.m/pixel). The SDP may or may not be
equivalent to spacing between raster lines and the MDP may or may
not be equivalent to the spacing between successive drop locations
along each raster line. The spacing between successive raster lines
may be defined as Secondary Drop Locations (SDLs), while spacing
between successive drop locations along each raster line may be
defined as Main Drop Locations (MDLs). Similar to SDPs and MDPs,
SDLs and MDLs may be defined in terms of drops per unit length or
drop spacing.
[0119] If SDP=SDL there is a one to one correspondence between data
and drop locations along the secondary scanning direction and the
pixel spacing is equal to that of the raster line spacing. If
MDP=MDL there is a one to one correspondence between data and drop
locations along the main scanning direction.
[0120] If SDL and/or MDL is larger than SDP and/or MDP,
respectively, more drops will need to be fired than that for which
data exists, thus each pixel will need to be used to control the
dropping of more than one droplet. The dispensing of these extra
droplets can be done either by dispensing the droplets at
intermediate points between the centers of successive pixels (i.e.
intermediate dropping, "ID") or alternatively, directly on top of
pixel centers (i.e. direct dropping, "DD"). In either case this
technique is called "overprinting" and results in faster build up
of material and eases mechanical design constraints involving
maximum scan speeds and acceleration rates since the same Z-build
up can occur while moving the print head and/or object more slowly.
The difference in ID overprinting versus non-overprinting, or DD
overprinting, is depicted in FIGS. 16a to 16d. FIG. 16a depicts a
single drop 60 being deposited and an associated solidified region
62 surrounding it when the print head is moving in direction 64. On
the other hand, FIG. 16b depicts the same region being cured but
using the ID overprinting technique where two drops 60 and 66 are
deposited in association with the single data point when the head
is moving in direction 64. The deposition zone filled by the two
drops is depicted by region 68. FIG. 16c shows a similar situation
for a four drop ID overprinting scheme wherein the drops are
indicated by numerals 60, 70, 66 and 72 and the deposition zone is
depicted by 76 and wherein the scanning direction is still depicted
by numeral 64. FIG. 16d depicts a similar situation for a line of
pixels 78, 80, 82, 84, 86 and 88 wherein numeral 90 depicts the
length of the deposition zone without overprinting and the numeral
92 depicts the length of the deposition zone when using a four drop
ID overprinting technique. The above can be generalized by saying
that ID overprinting adds from approximately 1/2 to just under 1
additional pixel length to any region wherein it is used. Of
course, the more overprinting drops that are used, the more
vertical growth a pixel region will have.
[0121] If SDL and/or MDL is less than SDP and/or MDP, respectively,
drops will be fired at fewer locations than those for which data
exists, at least for a given pass of the print head. This data
situation may be used to implement the offset pixel and/or
non-uniform sized pixel techniques discussed above.
[0122] An N row by M column grid is depicted in FIG. 7. As shown,
the rows in the grid are labeled as R(1), R(2), . . . , R(N), while
the columns in the grid are labeled as C(1), C(2), . . . , C(M).
Also shown are the pixels making up the grid. These are labeled as
P(1,1), P(1,2), . . . , P(M,N).
[0123] To build a cross-section, the bit map is first loaded with
data representative of the desired cross-section (as well as any
supports which are desired to be built). Assuming, as with some
preferred embodiments, a single build and support material is being
used. If it is desired to deposit material at a given pixel
location, then the memory cell corresponding to that location is
appropriately flagged (e.g. loaded with a binary "1") and if no
material is to be deposited an opposite flag is used (e.g. a binary
"0"). If multiple materials are used, cells corresponding to
deposition sites are flagged appropriately to indicate not only
drop location sites but also the material type to be deposited. For
ease of data handling, compressed data defining an object or
support region (e.g. RLE data which defines on-off location points
along each raster line as described in concurrently filed U.S.
patent application Ser. No. 08/722,326, now U.S. Pat. No.
5,943,235) can be Booleaned with a fill pattern description (e.g.
Style file information as described in U.S. Pat. No. 5,943,235) to
be used for the particular region to derive a final bit map
representation used for firing the dispensing jets. The actual
control of the jets may be governed by a subsequently modified bit
map which contains data which has been skewed or otherwise modified
to allow more efficient data passing to the firing control system.
These considerations are discussed further in the U.S. Patent
Application based on 3D Systems' U.S. Pat. No. 5,943,235. The
raster lines making up the grid are then assigned to individual
orifices in the manner described earlier. Then, a particular
orifice is directed to fire or not at firing locations
corresponding to desired drop locations or pixel locations
depending on how the corresponding cells in the bit map are
flagged.
[0124] As discussed above, the print head 9 is capable of
depositing droplets at many different resolutions. In some
preferred embodiments of the present invention SDP=SDL=300 pixels
and drops per inch. Also in some preferred embodiments, MDL is
allowed to take on three different values while MDP remains fixed
1) MDL=300 drops per inch and MDP=300 pixels per inch; 2) MDL=600
drops per inch; and MDP=300 pixels per inch or 3) MDL=1200 drops
per inch and MDP=300 pixels per inch. When the MDL to MDP ratio is
greater than one, the extra drops per pixel are made to occur at
intermediate locations (ID overprinting) between the centers of the
pixels. With the currently preferred print head and material, the
volume per drop is about 80 to 100 picoliters which yields roughly
drops having a 2 mil (50.8 .mu.m) diameter. With the currently
preferred print head, the maximum frequency of firing is about 20
kHz. By way of comparison, a firing rate of 1200 dpi at 13 ips
involves a firing frequency of about 16 kHz, which is within the
permissible limit.
[0125] In some preferred embodiments, build styles are defined
separately from object data for ease of data manipulation, transfer
and memory loading. In this regard, as noted above, data
descriptive of the object is Booleaned (e.g. intersected) together
with information descriptive of a build style, on a pixel by pixel
basis, to yield a pixel by pixel representation of the deposition
pattern at any given location. For example, if a completely solid
pattern is to be dispensed in two passes (e.g. a two step pattern),
the object data would first be Booleaned (e.g. intersected) with a
first build style pattern representing the portion of the pixels at
which drops are to be deposited (or for ease of terminology we may
say "exposed" in analogy to the selective solidification that is
used in photo-based stereolithography). The resultant modified
pixel data could thereafter be used to control jet firing. Next,
the object data would be Booleaned (e.g. intersected) with the
complementary build style pattern to yield modified pixel data for
controlling a second firing of the jets. In other preferred
embodiments, object data and support data can be immediately
correlated to build style data upon its derivation. In further
preferred embodiments, build style information could also include
pixel shifting information, pixel sizing information, overprinting
information, scan direction preferences for depositing on each
pixel location, planarization direction and rotational preferences,
and the like. The build styles described herein enhance system
performance by: 1) enhancing build speed; 2) enhancing accuracy of
the formed object; 3) enhancing surface finish; 4) reducing stress
in the object and/or distortion of the object; or 5) a combination
of one or more of these simultaneously.
[0126] A significant problem with Selective Deposition Modeling
systems involves ensuring the reliability of material deposition
and more particularly of achieving uniform thickness of deposited
cross-sections. Another problem involves achieving a uniform
thickness for all build styles. In ink jet systems this reliability
problem can take the form, inter alia, of misfiring or non-firing
jets. In a multijet system, further problems exist regarding
non-uniformity of jet firing direction, non-uniformity of dispensed
volume between jets, and to a lesser extent, non-uniformity of
dispensed volume from a single jet over time.
[0127] The problem of non-uniformity of cross-section thickness can
also result from other phenomena as well. As an example, once a
droplet leaves a jet there is a time of flight before the droplet
encounters the working surface. When leaving the jet, the drop is
fired with an initial downward velocity component away from the jet
but since the jet is moving in the main scanning direction the
droplet also has a horizontal velocity component. Once the droplet
leaves the jet it is subject to various external and internal
forces including gravity, viscous drag forces and surface tension.
These initial conditions and forces in turn lead to the conclusion
that the droplet may not, and probably will not, land directly on
the working surface below the position from which it was fired.
Instead the droplet will land somewhat away from this theoretical
drop point, typically in the direction of travel of the print head.
In other words the firing location and impact (or drop) location
will not have the same XY coordinates but instead will be shifted
one from the other. The shift in horizontal distance that occurs
depends on the above noted factors but also on the distance between
the orifice plate 10 and the vertical position (e.g. "Z" position)
of the working surface at each horizontal location (e.g. X and/or Y
position). As noted above variations in vertical position can occur
for a number of reasons. For example, variations can result from
differences in geometry between different portions of a
cross-section (more or less material spreading results in less or
more deposition thickness). As another example, variations can
result from the temporal ordering of deposition for a given spatial
pattern (previously deposited material on an adjacent pixel site
can limit the ability of the material to spread in that
direction).
[0128] As noted previously, the preferred system for embodying this
invention utilizes planarization to bring each deposited
cross-section to a uniform height wherein the net layer thickness
results from the difference in Z-level between the planarization
levels of two consecutive layers. In turn, if it is desired that
the planarization step form a completely smooth and uniformly
leveled layer, the Z increment between planarizations must be at or
below the minimum deposition/build up thickness for each point on
the entire layer. If one jet is weakly firing (or not firing), the
minimum thickness build up can result in net layer thicknesses much
smaller (i.e. near zero or zero) than desired and therefore much
longer build times than desired. Several techniques for dealing
with these deposition/build up problems are described herein. Other
preferred embodiments might involve the use of planarization on
periodic layers instead of on every layer. For example
planarization may be used on every second, third, or other higher
order spaced layer. Alternatively, determination of which layers or
portions of layers to planarize may be based on object
geometry.
Time of Flight Correction:
[0129] As noted above, one difficulty in ensuring that the droplets
strike the desired locations on the working surface involves the
time that the droplets are in flight (i.e. the time of flight of
the droplets). If the times of flight were always the same and if
the direction and amount of offset were always the same there would
be no time of flight issue since the only effect would be a shift
between firing coordinates and deposition coordinates. However,
when forming three-dimensional objects it is typically desirable to
jet material when the head is traveling in both the positive and
negative main scanning directions (and may even involve, for
example, alternating the definitions of main and secondary scanning
directions). This results in a change in offset direction (e.g.
reversal of offset direction) between scans due to relative
movement occurring in different directions (e.g. opposite
direction). This problem can be readily addressed by causing firing
signals to occur before the head actually reaches the point
directly above the desired deposition site. This correction to
firing time is known as the "time of flight correction". The time
of flight may be corrected by utilization of a correction factor
applied to scanning in each direction separately or alternatively a
single correction factor may be used to bring deposition from one
scanning direction into registration with the uncorrected scans
made in the other direction. The time of flight correction may be
implemented in a number of ways. One way, for example is by
appropriately defining the initial firing location (X position) at
the beginning of each raster line, which initial firing location
will be used to set the firing locations for all other pixels along
the raster line.
[0130] FIGS. 27a-27e illustrate the relationships between firing
location, drop location, and time of flight wherein like elements
are referenced with like numerals. FIG. 27a illustrates the
situation where firing locations 404a and 404b are both coincident
with desired drop location 402 (i.e. no time of flight correction
factor is used). Element 404a represents the firing location when
the head is passing in the positive X-direction, represented by
element 406a, and element 404b represents the firing location when
the head is passing in the negative X-direction, represented by
element 406b. Elements 408a and 408b represent the nominal path
followed by the droplets after leaving firing locations 404a and
404b, respectively. The nominal paths 408a and 408b direct the
droplets to actual drop locations 410a and 410b, where the droplets
impact the surface and form impacted droplets 412a and 412b. The
crossover point (i.e. focal point) for the droplets fired, while
scanning in both directions, is depicted with numeral 414. The
plane defined by the crossover points for the entire layer may be
called the focal plane. Elements 416a and 416b represent the time
of flight factor used in terms of an X-displacement between the
firing locations and the desired drop location. Whether or not the
actual drop locations match the desired drop location determines
the appropriateness the correction factor. In FIG. 27a it can be
seen that the droplets are moving in diverging directions and that
the impacted droplets do not overlap at the working surface
resulting in a minimal build up in Z and inaccurate XY placement of
material. FIG. 27b represents the situation where a small time of
flight correction factors 416a and 416b are used which result in a
focal point located above the desired working surface and in a
closer spacing of the impacted droplets 412a and 412b as compared
to that depicted in FIG. 27a. If the time of flight correction were
any larger, Z build up would be increased due to the overlap or
superposition of impacted droplets 412a and 412b. FIG. 27c
represents a situation where the time of flight correction factors
used result in the most accurate placement of impacted droplets
412a and 412b (assuming the thickness of impacted droplet 412a is
small compared to the drop distance 418 and that the angle of
incidence is not too large). If optimal time of flight correction
is based on maximum Z accumulation then FIG. 27c depicts the
optimal situation. FIG. 27d represents the situation where the time
of flight correction factors 416a and 416b are slightly larger than
those used in FIG. 27c but still result in Z-accumulation based on
the superposition of both droplets. The X-direction placements of
the droplets are still reasonably accurate and the focal point 414
of dispensing is somewhat below the desired working surface (and
actual working surface). FIG. 27e represents the situation where
even larger time of flight correction factors are used such that
Z-accumulation is reduced to a minimal amount and where the focal
point is even further below the desired working surface.
[0131] If drag effects and gravitational effects on flight time are
ignored, the time of flight correction value (time) would be equal
to the distance (length) separating the orifice from the working
surface divided by the downward velocity (length/time) at which the
droplet is dispensed. However, it is believed that drag is an
important factor. For example, in some preferred embodiments print
head scanning speed is approximately 13 inches per second, the
distance from the orifice plate to the working surface is
approximately 0.020 inches, and the initial vertical firing speeds
are believed to be on the order of 200 to 360 inches per second. If
drag or other frictional forces are ignored, under these initial
conditions, a shift between firing locations and drop locations of
approximately 0.8 to 1.3 mils would be expected. However, under
these conditions, in practice shifts in the main scanning direction
between the firing location and drop location of approximately 2
mils have been observed.
[0132] The appropriate correction value can be readily determined
empirically by attempting to deposit droplets at a single X
location when scanning in both directions and reiterating the
experiment with different correction values until the two drops
land at the same point. As noted above, in some preferred
embodiments the most appropriate time of flight correction value is
the one for which the droplets hit the same position. In terms of
the above example, if drag forces are ignored, time of flight
correction factors of approximately 60 to 100 .mu.S would be
expected. When in practice correction factors of approximately 150
to 200 .mu.S have been found to be more appropriate.
[0133] In other preferred embodiments the optimal time of flight
correction factor is not set at a value which yields the most
accurate targeting (i.e. the focal point is not at the working
surface) but instead is set at a value which would yield most
accurate targeting some distance below the actual working surface
(i.e. the focal point is located below the working surface). These
embodiments may be called "off surface targeting" embodiments. In
this context, most accurate targeting is considered to occur when
vertical accumulation rate is the greatest and probably when the X
position is most precisely impacted. FIG. 27d depicts an example of
targeting for these off surface targeting embodiments. These off
surface targeting embodiments are believed to be particularly
useful when building is to occur without the use of additional
components for maintaining the desired and actual working surface
at the same level (e.g. without a planarizer or without additional
elements such as a surface level detection device and adjustment
mechanisms or schemes).
[0134] A characteristic of these off surface targeting embodiments
is that Z-accumulation is self-correcting or self-compensating. As
long as the Z-increments between deposition of successive layers
are within an appropriate range and the deposition pattern allows
horizontal spreading of dispensed material instead of only vertical
accumulation, excess Z-accumulation on one layer causes a reduction
in Z-accumulation on one or more subsequent layers causing the net
accumulation to maintain the focal point somewhat below the actual
working surface. On the other hand, again as long as Z-increments
between deposition of successive layers is within an appropriate
range and the deposition pattern allows horizontal spreading of
dispensed material instead of only vertical accumulation, too
little Z-accumulation on one layer causes an increase in
Z-accumulation on one or more subsequent layers thereby causing net
accumulation to maintain the focal point somewhat below the actual
working surface. The preferred Z-increment range is discussed
further below.
[0135] This self-correcting aspect can be understood by studying
and comparing FIGS. 27c, 27d and 27e. When deposition begins (e.g.
at the platform) the time of flight correction factor(s) are chosen
such that the focal point is somewhat below the actual working
surface as depicted in FIG. 27d (i.e. the focal point should be set
at an appropriate position such that the situations depicted in
FIGS. 27c and 27e do not occur). If when forming the first layer,
too little material is deposited, for the given Z-increment being
used, the actual surface will be lower as compared to the
repositioned focal plane (but will still be above it as long as the
Z-increment was not too large). This results in a more optimally
focused deposition when forming the next layer, this in turn
results in an increase in deposition thickness as depicted in FIG.
27c. If the net Z-accumulation resulting from depositing the second
layer is still too low (as compared to the two Z-increments made),
then the next layer when being deposited will have an actual
surface which closer to the optimal focus plane than the original
surface was. This closer approach to optimal positioning results in
increased Z-accumulation which will again drive the net accumulated
thickness toward that required by the Z-increments. On the other
hand, if net accumulation from depositing the second layer is
greater than that dictated by the two Z-increments, then the actual
working surface will be further away from the focal plane and less
Z-accumulation, upon forming the next layer, will occur thereby
driving the net accumulation toward the amount required by the
Z-increments. This is the situation depicted in FIG. 27e.
[0136] When the focal plane is appropriately below the actual
working surface, when the z-increment amount is appropriately
selected to approximately match deposition rates, and when
objects/supports are being formed in a non-solid manner (not all
pixel locations are directly deposited on, the system is stabilized
and both supports and objects can be formed with accurate vertical
dimensions without the explicit need of a planarizer. Of course a
planarizer can still be used if desired. For optimal operation of
these embodiments it is preferred that the Z-increment should be
selected to be between the average amount accumulated per layer
during optimal targeting (e.g. FIG. 27c) and the average amount
accumulated when no superposition occurs (e.g. FIG. 27e). It is
further preferred that layer thickness be significantly less than
the distance that separates the plane of optimal focus (e.g. FIG.
27c) from the plane where superposition no longer occurs (e.g. FIG.
27d).
[0137] As noted above, in some of these embodiments objects may be
formed in such a manner as to allow regions for material to spread
horizontally instead of just accumulating vertically, based on the
level of targeting optimization, and thereby allowing self
correction of Z-accumulation. One such embodiment might involve the
forming the object as a combination alternating solid layers and
checkerboard layers. Other such embodiments might involve the
formation of solid outward facing surfaces and checkerboard, offset
checkerboard, or other open structures in internal object regions.
Other appropriate building patterns can be determined empirically
by building and analyzing test parts.
[0138] In some of these offset surface targeting embodiments, the
most preferred initial target surface/focal plane position is
selected to be approximately in the middle of the situations
depicted in FIGS. 27c and 27e. One way of accomplishing this is to
ignore the hypothetical focal points and instead focus on time of
flight values. The time of flight correction values may be selected
so that they are greater than the optimal time of flight correction
values (as discussed above) and less than the time of flight
correction values which yields immediately adjacent but
non-overlapping (i.e. non-superimposed) impact zones. Most
preferably the selected time of flight values would be taken as
approximately the average of these two extremes.
[0139] Some offset surface targeting embodiments might be used to
simultaneously form different portions of objects and/or supports
such that their upper surfaces are intentionally at different
heights after formation of any given layer. These different height
embodiments might benefit from utilization of data manipulation
techniques, like the SMLC techniques, discussed in previously
referenced U.S. Pat. No. 5,999,184 as well as some of the other
previously referenced U.S. Patents and applications.
[0140] In addition to the above noted time of flight issues, other
issues arise that can be corrected using modified time of flight
correction factors. For example, when using ID overprinting
techniques to cause more build up, features on scan lines which are
scanned in opposite directions will lose alignment since the
feature will be extended in one direction on one line and in the
other direction on another line. This situation is depicted in
FIGS. 17a and 17b. FIG. 17a depicts two points 60 and 100 belonging
respectively to scan lines traversed in directions 64 and 104.
Regions 62 and 102 depict the extents of deposited material
associated with points 60 and 100, respectively. FIG. 17b depicts
the same points 60 and 100 where jetting occurs using four times
overprinting (i.e. four droplet depositions per pixel). Extents of
deposition are depicted with numerals 76 and 106 respectively. As
can be seen, due to the different directions of overprinting,
registration between the physical features on the two lines is
lost. The above mis-registration can be corrected by an additional
time of flight correction factor which can be empirically, or
possibly theoretically determined so as to cause realignment of
features on different scan lines. Of course this form of correction
does not account for any extra length added to object features
along the scanning lines.
[0141] A different form of correction that can avoid both problems
is proposed which involves recognition that a given pixel is not
bounded on its far side, in the scanning direction, by an adjacent
pixel that also calls for material deposition. Based on this
recognition, no overprinting is used on such an unbounded pixel. As
another alternative, the extra line length might be compensated for
by using a form of drop width compensation similar to line width
compensation used in photo-based stereolithography and as described
in the previously referenced U.S. Pat. Nos. 5,854,748 and 5,870,307
but applied only to the points along each scan line representing a
transition from deposition to no deposition. As an approximate
correction these "terminal points" could simply be deleted from the
deposition pattern as they will be in the range of 1/2 to fully
covered by the use of ID overprinting of immediately adjacent
pixels. Another variant involves the use of shifted time of flight
correction data to implement subpixelling deposition.
[0142] The time of flight correction factors may also be used in
variant manners for somewhat opposite purposes to those described
above. In these embodiments, time of flight correction factors may
be used to deposit material at intermediate pixel (i.e. subpixel)
locations for implementation of enhanced building techniques. These
enhanced building techniques might involve formation of down-facing
surfaces, formation and placement of supports, enhanced vertical
build up of material, enhanced resolution, and the like. In
preferred embodiments, enhanced object formation may be achieved in
a single pass or multiple pass implementations.
Droplet Width Compensation
[0143] In some situations it may be desirable to modify the object
data by performing droplet width compensation (i.e. deposition
width compensation). Compensation (by offsetting inward toward
solid one or more full pixel widths) can be used to achieve
enhanced accuracy if the drop width is at least somewhat greater
than the pixel width and/or length. This technique may be used in
combination with any of the embodiments described above or any
embodiments described herein after. As the drop width approaches or
exceeds twice the pixel width (and/or length) better and better
accuracy can be obtained by a single or multiple pixel offset.
Droplet width compensation may be based on techniques like those
disclosed in U.S. Pat. Nos. 5,854,748 and 5,870,307. Alternatively
they may involve pixel based erosion routines. In some embodiments
the pixel based erosions might involve multiple passes through a
bit map wherein "solid" pixels meeting certain criteria would be
converted to "hollow" pixels.
[0144] Some embodiment might involve the following steps wherein
each edge of the bit map is: 1) In a first pass through the bit map
all "solid" pixels which are bounded on their right side by a
"hollow" pixel are converted to "hollow" pixels; 2) In a second
pass all "solid" pixels which are bounded on their left side by a
"hollow" pixel are converted to "hollow" pixels; 3) In a third pass
all "solid" pixels which are bounded on their upper side by a
"hollow" pixel are converted to "hollow" pixels; and 4) In a fourth
pass all "solid" pixels which are bounded on their lower side by a
"hollow" pixel are converted to "hollow" pixels. Other embodiments
might change the order of steps (1) to (4). If more than a one
pixel erosion is required, steps (1) to (4) can be repeated as
multiple times until the correct amount of reduction is achieved.
These embodiments can perform a reasonable droplet width
compensation; however, they suffer from the short coming that
pixels in solid corner regions (whether an object corner or an
object edge that does not run parallel to either the X or Y axis)
are removed at a faster rate than pixels in which represent
boundary regions that are parallel to either the X or Y axis.
[0145] Other embodiments which attempt to address these
differentials in erosion rate might involve steps as follows: 1) In
a first pass through the bit map all "solid" pixels which are
bounded on their right side by a "hollow" pixel and all other sides
by "solid" pixels are converted to "hollow" pixels; 2) In a second
pass all "solid" pixels which are bounded on their left side by a
"hollow" pixel and on all other sides by "solid" pixels are
converted to "hollow" pixels; 3) In a third pass all "solid" pixels
which are bounded, on at least their upper side, by a "hollow"
pixel are converted to "hollow" pixels; and 4) In a fourth pass all
"solid" pixels which are bounded, on at least their lower side, by
a "hollow" pixel are converted to "hollow" pixels. Other
embodiments might change the order of steps (1) to (4) or the
conditions on which conversion will be based. If more than a one
pixel erosion is required, steps (1) to (4) can be repeated as
multiple times until the correct amount of reduction is achieved.
These embodiments do a better job of minimizing excess reduction in
corner regions.
[0146] Other embodiments, might involve setting erosion conditions
based on whether or not two, three or all four sides of a pixel are
bounded by "hollow" pixels. Other embodiments may vary the erosion
conditions depending on how many times the bit map has been passed
through. Other embodiments may use a combination of erosions and
Boolean comparisons with original cross-section or other partially
compensated bit maps to derive final bit map representations of the
pixels to be exposed. Numerous other embodiments and algorithms for
eroding pixels while emphasizing the reduction or maintenance of
certain object features will be apparent to those of skill in the
art in view of the teachings herein.
[0147] In situations where X and Y pixels dimensions are
significantly different, droplet width compensation may only be
necessary along one axis instead of both axes. In these situations,
embodiments similar to those described above may be used wherein
only a portion of the steps will be performed per erosion. It is
anticipated that deposition width compensating schemes can also be
utilized using subpixel offset amounts in either one or both of the
X and Y dimensions.
Randomization
[0148] A technique (method and apparatus) known as randomization
can be employed in the build process. This technique may be used in
combination with any of the embodiments described above or any
embodiments described herein after. According to this technique,
the manner of dispensing material at each location for two
consecutive cross-sections is varied. This can lead to a more
uniform build up of material across a layer (i.e. lamina) resulting
in the ability to potentially use thicker layers, thus improving
build time. This technique also minimizes the effects from any
single jet or plurality of jets that may not be properly firing.
The varying of deposition can occur in several ways. For example
variation may occur by: 1) varying the jet which deposits material
onto a given portion of a layer relative to the jet that deposited
material on the corresponding portion of the immediately preceding
layer; 2) varying the temporal order or spatial order of dispensing
onto any given portion of the layer relative to any other portion
of the layer; and 3) a combination of these, such as varying the
main scanning orientation or direction and/or varying the secondary
scanning orientation or direction. The varying of deposition from
layer to layer can occur in a totally random manner or it can occur
in a periodic or planned manner. A similar technique has been used
in photo-based stereolithography though for a completely different
purpose (see Alternate Sequencing in previously referenced U.S.
Pat. No. 5,711,911).
[0149] Specific embodiments for varying the deposition will now be
given. The presently preferred randomization technique maintains
the orientation of the main and secondary scanning directions but
uses a different dispenser (e.g. jet) to deposit material along
corresponding scanning lines between two layers. In other words, a
first dispenser is used to scan a particular main scanning line on
a first layer and a second dispenser may be used to scan that
particular main scanning line on a subsequent layer (the one
immediately above the particular scanning line on the first layer).
In some preferred embodiments, a particular scan line is exposed
(i.e. deposited on), from layer-to-layer, using a different jet
until 96 layers have been deposited and each of the 96 jets has
deposited on the particular scan line, after which the process
repeats. These embodiments are examples of "full head"
randomization. In other preferred embodiments, "half head"
randomization is preferred. Half head randomization can reduce the
number of passes that must be made over the any cross-section
depending on the object geometry. Based on building with the
presently preferred 96 jet head, half head randomization involves
scanning over any given location with randomized dispensing
occurring from either jets 1 to 48 or jets 49 to 96.
[0150] To explain the full head randomization embodiments in more
detail, reference is made to FIGS. 4a and 6. For a particular
layer, orifice 10(1) might be used to trace scan lines R(1)-R(8);
orifice 10(2), lines R(9)-R(16); orifice 10(3), lines R(17)-R(25);
orifice 10(4), lines R(26)-R(33), etc. On the next layer, however,
these assignments are changed such that a given orifice does not
trace the same scan line on the next layer. For example, the
following new assignments might be made: orifice 10(1), lines
R(257)-R(264); orifice 10(2), lines R(265)-R(272); orifice 10(3),
lines R(273)-R(280), etc.
[0151] Another embodiment might involve relatively rotating the
partially formed object and/or the print head by some amount (e.g.
30.degree., 60.degree., or 90.degree.) between the deposition for
two layers so that the main and secondary scanning orientations are
changed from their previous orientations. This results in material
deposition on a current layer, (i.e. lamina), from any jet, mainly
occurring above material which was deposited by other jets on the
previous layer. This is depicted in FIG. 8 wherein the scan lines
associated with a first layer are depicted by lines R.sub.1(1),
R.sub.1(2), R.sub.1(3), R.sub.1(4), . . . , R.sub.1(N-3),
R.sub.1(N-2), R.sub.1(N-1), R.sub.1(N) while the scan lines
associated with a subsequent layer are depicted by line R.sub.2(1),
R.sub.2(2), R.sub.2(3), R.sub.2(4), . . . , R.sub.2 (N-3),
R.sub.2(N-2), R.sub.2(N-1), R.sub.2(N) which are rotated by
90.degree. with respect to the scanning lines of the first layer.
The amount of rotation may vary between subsequent layers or it may
be a constant amount. The angles may be chosen such that if the
rotation is continued for a sufficient number of layers, identical
jets will deposit material over identical scan lines where
deposition occurred on previous layers. Alternatively, the angles
may be chosen so that no identical jet to scan line redeposition
occurs.
[0152] Additional embodiments might involve changing the order of
progression from one scan line to another (in the secondary
scanning direction). This is depicted in FIG. 9, where for a first
layer the order of deposition of material on main scan lines begins
on the upper most scan line, R.sub.3(1) and proceeds to scan lines
R.sub.3(2), R.sub.3(3), . . . , R.sub.3(N-2), R.sub.3(N-1), and
ends with lower most scan line R.sub.3(N). The order of progression
of scan lines is depicted by arrow R.sub.3P. The deposition of
material on scan lines for a subsequent layer begins on the lower
most scan line, R.sub.4(1) and proceeds to scan lines R.sub.4(2),
R.sub.4(3), . . . , R.sub.4(N-2), R.sub.4(N-1), and ends with upper
most scan line R.sub.4(N). The order of progression of scan lines
on this subsequent layer is in the opposite direction to that of
the lines on the first layer and is depicted by arrow R.sub.4P.
[0153] Additional embodiments are depicted in FIGS. 10a and 10b,
wherein the direction of scanning along corresponding scan lines is
reversed between two subsequent layers. FIG. 10a depicts the
scanning directions for scan lines on a first layer wherein scan
lines R.sub.5(1) and R.sub.5(3) are scanned from left to right and
scan line R.sub.5(2) is scanned from right to left. FIG. 10b
depicts that the scanning directions are reversed on a subsequent
layer wherein scan lines R.sub.6(1), R.sub.6(2), and R.sub.6(3)
overlay R.sub.5(1), R.sub.5(2), and R.sub.5(3), respectfully, and
wherein scan lines R.sub.6(1) and R.sub.6(3) are scanned from right
to left and scan line R.sub.6(2) is scanned from left to right.
[0154] Many other randomization patterns are possible including
combinations of the above described techniques. Depending on the
randomization technique chosen, the randomization process may cause
an overall increase in layer deposition time since it might result
in the need to perform additional main scanning passes. However,
this possible disadvantage is believed to be outweighed by the
enhancement in uniform layer build up. Additionally, since heat
removal is a significant problem when using elevated dispensing
temperatures (as used to make the material flowable), these extra
passes can be effectively used to allow additional cooling to occur
prior to deposition of a subsequent layer.
Drop Location Offsetting
[0155] As noted above some building techniques can be enhanced by
the use of offset scan lines and/or offsetting of drop locations
along scan lines. These offsetting techniques could be used in
combination with the above noted randomization techniques, though
it should be understood that corresponding lines and drop locations
on successive layers may be offset from one another. These
techniques may also be used in combinations with other embodiments
disclosed herein before or herein after. In some preferred
embodiments this offsetting may be up to 1/2 the line spacing or
drop spacing. One use of offset pixelling might involve deposition
of material on a down-facing portion of a cross-section so as to
aid in bridging the gap between adjacent support elements. In fact
the down-facing region may be cured in multiple passes wherein
progressive or alternating offset, between successive passes, are
used to bridge a wide gap between support elements. In these
embodiments any non-down-facing portion of the cross-section might
be exposed using one or more depositions and offset or non-offset
pixels and deposition in any down-facing portion might occur by
multiple depositions (or exposures) where pixels zones partly
overlap. The overall height of deposition, in preferred
embodiments, might be made uniform by trimming to an appropriate
level by planarizing.
[0156] In some embodiments, offsetting of pixels and therefore drop
sites might occur during support structure formation to enhance the
formation of arch-like supports, bridges, or branching supports
(e.g. like limbs of a tree). In some embodiments, offsetting of
pixels might occur during object formation to enhance building of
object sections which protrude a limited amount beyond the
boundaries of the immediately preceding object lamina. Protruding
supports and object portions can be formed without the use of
offset pixelling but it is believed that offset pixelling can be
useful to aid in the formation of such structures wherein less
material may slump into regions below the layers levels at which it
was dispensed.
[0157] Embodiments may involve the offsetting of pixels on every
layer or alternatively may involve the offsetting of pixels only on
periodic layers. In this last alternative, material is deposited
according to the same pixel positions over a number of layers.
According to this alternative, stabilization of overhanging regions
may be allowed to better occur by build up of multiple layers,
above an initial overhang, prior to attempting the formation of a
subsequent overhang.
[0158] The offsetting of pixels, e.g., to build branching supports
or tapering outward object structures, results in the formation of
structures which branch out over empty space. The extent of this
branching is limited to some thing less than one droplet width per
layer. Whether each layer extends beyond the boundary of its
immediately preceding layer, or whether multiple layers are built
above one another followed by periodic extensions beyond the
boundary of an immediately preceding layer, one can define an angle
of extension based on the average extension over a number of
layers. The maximum angle of extension depends, in part, on the
rate at which the material near and in the extended portion
solidifies, which in turn depends on the amount of material
dispensed near and in the extended portion. The layers can be built
at any angle wherein the material solidifies quickly enough and is
capable of supporting the next layer of material. In some preferred
embodiments, extension angles close to 30 degrees have been
achieved. It is believed that extension angles approaching or even
exceeding 45 degrees are possible.
[0159] Due to material cool down rates, it is preferred that the
formation of overhanging solid object portions be achieved in
multiple passes. In one preferred embodiment, the extension region
is deposited on one or more initial passes and the fully supported
regions are exposed on one or more subsequent passes. This
embodiment allows the material in the extension regions to cool and
solidify without the added delays that might be associated with
heat absorption from material dispensed in the interior regions. In
another preferred embodiment, the interior of the layer is exposed
initially and the extension regions are exposed in one or more
subsequent passes. This embodiment allows time for the material on
the interior portions to cool somewhat prior to dispensing the
extension regions thereby reducing the risk of the extension
material remaining flowable for too long a time. For a given set of
build parameters, the useable extension angles can be empirically
determined by building and examining test parts.
[0160] Offsetting of pixels may be used in combination with
multiple passes over a given portion of a layer so as to allow
build up of material around a given geometric feature in a
prescribed order and offset pattern. For example, offsetting may
occur on one side of a feature such that a fraction of a pixel
shift in position occurs away from that side of the feature, while
a different offset might be used so that the same fractional shift
might be obtained in the opposite direction on the opposite side of
the feature.
[0161] An alternative to offset pixelling is simply to build
objects using higher resolution data and associated build patterns
or styles that yield the desired drop density which may be less
than that provided inherently by the data but which still may yield
formation of solid structures or other desired features.
Scan Line Interlacing
[0162] Interlacing is another technique that can be used to enhance
object formation. As with all other embodiments disclosed herein,
the embodiments of this section are combinable with the other
embodiments disclosed herein. As discussed previously, if the head
is not oriented at the saber angle, the spacing between the jets is
not equal to the desired resolution and thus is not equal to the
desired spacing of main scanning or raster lines. As such, by its
nature, a form of main scan line interlacing must be used if it is
truly desired to deposit material along all main scan lines.
However, additional interlacing may be done for a number of reasons
(e.g. to enhance layer cooling and/or material build up).
[0163] A variety of scan line interlacing patterns can be used,
whether or not the print head is oriented at the saber angle,
whether or not the preferred raster scanning technique is used,
whether or not a vector scanning technique is used, whether or not
some other scanning technique or combination technique is being
used.
[0164] In one preferred embodiment, as previously described, the
head is oriented perpendicular to the main scanning direction and a
resolution of 300 scan lines per inch is used. In this
configuration successive jets are spaced {fraction (8/300)} of an
inch apart. The head is made to perform 8 main scans, the first 7
of which are followed by a secondary scan of width equal to the
spacing between the raster lines (raster width), and the eighth of
which is followed by a secondary scan of width equal to the
effective head width plus the raster width. Repetitions of the
above scanning pattern are made until the width incremented in the
secondary scanning direction has equaled or exceeded the width of
the building region.
[0165] Alternative embodiments could limit the X range of main
scanning to that which is sufficient to effectively cover the
working region required by the object, by the particular object
cross-section being scanned, by each segment of the object length
required to make the 8 closely spaced main scans, or by other
schemes which lead to a reduction in scanning time. Similarly, the
positioning along the secondary scanning axis could likewise be
limited to the width and position of the object, cross-section
being scanned, particular portion of a cross-section being scanned,
or the like. In preferred embodiments, the use of randomization may
increase the amount of indexing needing to be performed so that the
appropriate jets may trace the appropriate main scan lines. Other
embodiments may limit the main scanning to paths which actually
include active drop locations.
[0166] As a first preferred alternative interlacing technique
non-adjacent scan lines would be left unexposed after at least a
first pass whereafter on one or more subsequent passes the
intermediate lines would be exposed. In other preferred
embodiments, it is desired that intermediate raster lines be drawn
prior to depositing material on either adjacent raster line or
after depositing material on both adjacent lines. Examples of this
type of embodiment are depicted in FIGS. 11a, 11b, and 22a-22d.
FIGS. 11a and 11b depict the situation wherein every other line is
skipped on a first pass. FIG. 11a depicts four scanning lines
wherein two lines are to be exposed (i.e. the drop locations to be
used) on a first pass. FIG. 11b depicts the same four scanning
lines wherein the other two lines are to be exposed (i.e. the drop
locations to be used) on a second pass. Further examples of
interlaced patterns are shown in FIGS. 22a-22d. In these figures
two headed arrow 30 represents the main scanning direction, spacing
d.sub.r represents the spacing between successive raster lines, and
for clarity, the beginning points and end points of the lines are
shown offset although in practice the lines would have the same
beginning and ending points. FIG. 22a depicts a series of raster
lines to be scanned in the main scanning direction. FIG. 22b
depicts first raster lines 32 to be exposed on a first pass and
second raster lines 34 to be formed on a second pass according to
the example of FIGS. 11a and 11b. FIG. 22c depicts raster lines 32,
34, 36 and 38 to be exposed on first, second, third and fourth
pass, respectively. FIG. 22d depicts raster lines 32, 34, 36, 38,
40, and 42 to be exposed on first, second, third, fourth, fifth and
sixth pass, respectively. In the example of FIG. 22d other raster
line scanning orders could also be used while still ensuring that
when intermediate lines are deposited they are either not bounded
on either side or that they are bounded on both sides by previously
deposited adjacent raster lines. For example, other useful scanning
orders might be 32, 34, 38, 36, 40 and 42; 32, 36, 34, 40, 38 and
42; or the like.
[0167] In one preferred system, to fully implement these
embodiments in a generalized manner using a minimum number of
passes, an odd number of raster lines would need to exist between
the line scanned by one of the jets (e.g. a first jet),and the line
scanned by an adjacent jet (e.g. a second jet). In other words, the
number of d.sub.r spacing between successive jets would have to be
even; thereby requiring that two adjacent jets must be positioned
so as to scan raster lines M and M+N where M and N are integers and
N is even. In the case where the spacing between the jets is not
appropriate (e.g. not even), it is always possible to scan only
appropriate raster lines (e.g. those associated with every other
jet) in a first pass and then to expose the remaining scan lines in
one or more subsequent passes. As the width of deposition may be
significantly wider than the raster line spacing, other preferred
embodiments might not be based on the skipping of every other scan
line on a first pass, but instead be based on the selection of scan
lines for deposition (i.e. exposing) on the first pass such that
the lines of deposited material do not directly contact each other
and then filling in any skipped raster lines on one or more
subsequent exposures.
[0168] This first alternative interlacing technique can be fully or
approximately implemented even when the adjacent jets are
inappropriately positioned for the desired scan line resolution
(i.e. the jet positions and scan line resolution are such that an
even number of raster lines exist between the line scanned by one
of the jets and the line scanned by an adjacent jet). This may be
accomplished in at least three ways: 1) each jet is used to scan
every other raster line between its initial position and the
position of the line initially formed by the adjacent jet except at
least two adjacent raster lines to be scanned by each jet will be
left unexposed until at least a second pass when the remaining
raster lines will be exposed; 2) each jet is used to scan every
other raster line until it also scans the raster line adjacent to
the first line scanned by the adjacent jet whereafter the remaining
unexposed lines will be selectively exposed in a second pass;
and/or 3) only every other jet is used in the scanning process
thereby ensuring that an odd number of raster lines exist between
any two adjacent jets. In these embodiments, as well as the
previous embodiments, it is preferable to expose alternating lines
for the whole layer prior to beginning a second pass to expose the
intermediate lines; however it is possible to complete the exposure
of all scan lines between the starting points of some or all of the
adjacent jets prior to making even a first pass over other portions
of the layer.
[0169] Numerous other interlacing embodiments will apparent to
those of ordinary skill in the art who study this disclosure. For
example, interlacing with higher numbers of passes can be used or
interlacing wherein some contact occurs between lines exposed on a
first pass. Of course any combination of interlacing with the
previously described randomization techniques could also be used.
Further exposure of a subsequent layer may change the order of
scanning the various sets of lines and/or the scanning directions
of the lines themselves (e.g. reverse the order of scanning of the
first, second, and higher order sets). Further embodiments might
involve the completion of interlacing exposures for a first layer
while exposing regions during formation of one or more subsequent
layers.
Drop Location Interlacing
[0170] As with scan line interlacing, object formation may utilize
drop location interlacing along individual scan lines. In this
case, each scan line would be exposed by at least two passes
wherein a first pass would expose a number of drop locations and,
whereafter, on one or more subsequent passes, the remaining drop
locations would be exposed. As a two step (i.e. pass) example, on a
first pass every other drop site would be exposed while on a second
pass the intermediate drop sites would be exposed. This situation
is depicted in FIGS. 12a and 12b. FIG. 12a depicts four scanning
lines each with 9 drop locations wherein every other drop location
is to be exposed on a first pass while FIG. 12b depicts the same
lines and locations but instead depicts that only complementary
drop locations are to be exposed on a second pass. As a second two
step example every third site may be exposed on a first pass while
on a second pass both the intermediate sites, there between, would
be exposed. As a three step example, a first pass might expose
every fifth site beginning with the first site, then on a second
pass every fifth site would be exposed beginning with the third
site, and finally on a third pass every other site would be exposed
beginning with the second site.
[0171] As with all other embodiments disclosed herein, the
embodiments of this section are combinable with the other
embodiments disclosed herein.
[0172] In these interlacing techniques, successive scan lines may
be exposed using different or shifted interlacing patterns so that
two dimensional interlacing patterns may be developed (offset
pixelling could also be used). For example, a two step interlacing
pattern may be used on each scan line wherein the starting points
on successive lines are shifted by one pixel such that a
checkerboard first pass pattern is formed. FIGS. 13a and 13b
illustrate this example. FIG. 13a depicts the first pass
checkerboard pattern while FIG. 13b depicts the complementary
checkerboard pattern that is exposed on a second pass.
[0173] As with scan line interlacing, drop location interlacing may
complete all passes over single lines prior to exposing subsequent
lines though it is preferred that all lines be exposed with each
pass prior to initiating subsequent passes over partially exposed
lines. Furthermore, completion of all passes over portions of
single lines may be achieved prior to initiating exposure over
remaining portions of those lines.
[0174] A third interlacing technique involves feature sensitive
interlacing. In this technique the order in which a given drop site
is exposed depends on the geometry of the immediate cross-section
alone or on multiple cross-section geometries. Feature sensitive
interlacing may involve one or both of scan line interlacing and
drop location interlacing. For example, in a single layer
embodiment one may determine the boundary regions of the
cross-sections and ensure that the boundary zones are exposed on a
first pass. Some interior portions of the cross-section might also
be exposed on the first pass or alternatively exposure of all
interior portions may be delayed until one or more subsequent
passes are made. For example, the interior portions may be exposed
using a checkerboard interlacing pattern on a first pass in
combination with all boundary regions also being exposed on the
first pass. Then on a second pass the remaining interior portions
would be exposed. It is also possible that a wide boundary width
could be defined for exposure on a first pass so that more than a
one-drop site width border may be placed around the cross-section
prior to performing subsequent passes. This wide boundary region
might be implemented using erosion routines such as those described
above in association with Droplet Width Compensation. As an
additional alternative, one may focus on ensuring that only one of
scan line boundary sites or drop location boundary sites
(boundaries along lines in the secondary scanning direction) are
exposed on the first pass. As a further alternative, internal
regions may be exposed in whole or in part prior to dispensing
material in boundary regions. It is believed that dispensing
boundary regions first might lead to improved build-up in the
vertical direction and that exposing boundary regions last might
lead to improved horizontal accuracy of the object. An even further
alternative might involve the dispensing of a near boundary region
initially, followed by the dispensing of deeper internal regions of
the cross-section and finally followed by dispensing of the outer
cross-sectional boundary itself.
[0175] Examples of a multi-cross-sectional feature sensitive
interlacing technique might involve initially exposing those
locations which form part of the present cross-section but which
were boundary or solid internal object regions on the previous
cross-section. The boundary and solid internal regions on the
previous cross-section might include boundary regions and solid
internal regions of support structures as well as object
structures. In using this embodiment deposition in at least
critical (i.e. important) down-facing object regions does not occur
on the first pass unless those down-facing regions are actually
supported by a structure of some nature (e.g. a support column
directly below). In one or more subsequent exposures, material is
dispensed to form unsupported down-facing features. Since the
deposition width is typically wider than the pixel width, it is
more likely that a droplet which is dispensed to land at a pixel
location adjacent to previously dispensed material on that
cross-section, the droplet will strike and hopefully adhere to the
neighboring deposited material as opposed to continue falling
downward to a cross-section below that for which is was intended.
Furthermore, since in preferred embodiments support structures are
typically no more than one pixel in separation, when exposure of
unsupported down-facing regions occur the dispensed material will
more likely be wedged between material already dispensed on the
present layer as opposed to being wedged between material dispensed
on a previous layer. However, since droplet diameter is typically
less than deposition diameter (i.e. impacted droplet diameter) and
since it may be smaller than pixel width, material deposited at an
adjacent pixel location may not sufficiently extend into the path
of a falling droplet so as to ensure a collision and stopping of
the particle.
[0176] In another preferred embodiment, the drop locations would be
shifted by a fraction of a pixel width (preferably approximately
1/2 a pixel width) along the main and/or secondary scanning
directions (preferably both) when dispensing unsupported
down-facing regions and preferably adjacent regions such that a
droplet is more likely to be at least partially supported by
previously dispensed material than if droplets were deposited in
perfect alignment. It is preferred that droplets over partially
unsupported regions be dispensed in a subsequent pass from those
dispensed over fully supported regions. However, it is possible to
rely solely on the overlap with the previous cross-section (and not
any additional benefit associated with adhesion to material
previously dispensed on the given cross-section) in ensuring
reasonable vertical placement of the droplets in at partially
unsupported regions. In this embodiment at least the support
regions (e.g. columns) on the current layer would not be shifted.
This ensures that registration from layer to layer occurs. It is
further preferred that wide gaps be closed by progressively working
deposition locations inward (i.e. multistage) from supported sides
of the gap using multiple passes over the cross-section, wherein
each pass is partially offset from the immediately preceding pass
to ensure adequate overlap of droplets so as to limit any material
placement beyond the required vertical level. Further, in one
preferred embodiment, Simultaneous Multiple Layer Curing
Techniques, as described in U.S. Pat. No. 5,999,184, are used in
order to offset critical down-facing data up one or more layers so
that upon deposition material forming the down-facing layer will be
located at the correct level.
[0177] An example of this multistage horizontally and vertically
offset embodiment using a 1/2pixel horizontal offset and 1 layer
thickness vertical offset is shown in FIGS. 23a-23h. FIG. 23a
depicts a side view of an object 120 to be formed. FIG. 23b depicts
object 120 as it would normally be divided into layers 122, 124,
126, 128, and 130. FIG. 23c depicts object 120 as it is to be
divided into layers 122, 124, 126, 128', and 130'. Layer 128' is
different from 128 in that the down-facing portion of the layer has
been removed as it is anticipated that it will be created during
deposition of the material on the next layer using a series of
successively offset exposures. Layer 130' is similar to layer 130
except that a different deposition pattern might be used in its
formation. FIG. 23d again depicts layers 122, 124, 126 and 128' but
in addition depicts deposition locations, or pixel positions,
132-137 at which material may be deposited during formation of
layer 130'. FIG. 23e is similar to FIG. 23d except that instead of
showing drop locations 132-137, drop locations 140-146 are shown.
As can be seen from the relative positions of the drop locations,
locations 132-137 and 140-146 are offset from each other by 1/2 a
pixel width. FIG. 23f depicts the deposition pattern formed from a
first pass of the print head in forming layer 130'. Droplets 150,
151, 152, and 153 are deposited at drop locations 141, 145, 142,
and 144, respectively. It can be seen that droplets 152 and 153
were only partially supported by layer 128' and that as a result it
is assumed they will partially extend (as depicted) into the region
originally belonging to layer 128. FIG. 23g depicts the deposition
pattern from the first pass in forming layer 130' as well as
additional material deposited in a second pass. Region 160 and 162
were deposited on the first pass and were represented in FIG. 23f
as regions 150, 152, 151 and 153. The deposition on the second pass
occurs according to the pixel arrangement depicted in FIG. 23d.
Droplets 155 and 156 are deposited at drop locations 132 and 137.
In practice, the dispensing of droplets 155 and 156 would initially
result in excess material being applied over a portion of regions
160 and 162 but this excess would be trimmed off during the
planarization process. Droplets 157 and 158 are deposited at drop
locations 134 and 135 but since these locations are not fully
bounded from below by material previously deposited, it is assumed
that a portion of the material dispensed will extend downward into
the region originally part of layer 128. The offset dispensing of
droplets 152, 153, 157 and 158 results in the formation of the
down-facing portion of layer 128 which was removed from layer 128'.
In a third and final pass, droplet 164 is deposited onto drop
location 143 to complete formation of layer 130'.
[0178] In other preferred embodiments various aspects to the above
example could be changed. For example, the extension of material
into lower layer regions (assumed to occur when droplets or drop
locations are only partially supported) could take on values other
than the 1 layer thickness extension described. The extension may
be less than 1 layer thickness or at least different from an
integral number of layer thickness. Maybe the extension would be an
integral number of layer thicknesses (e.g. 2 to 5 layer thickness
or more). In such a case, for most accurate formation, it would be
desired to have the initial object representation transformed into
a modified representation, as described in U.S. Pat. No. 5,999,184,
(either before or after generation of cross-sectional data) so that
when material is dispensed according to the modified
representation, the bottom of the down-facing feature is properly
located. Other variations might use geometry based deposition, in
multiple passes, along with different offset values such as 1/4 of
a pixel (so that 3/4 of the drop zone would be unsupported) or 3/4
of a pixel (such that only 1/4 of the drop location would be
unsupported). These different offset amounts might lead to more
control over the amounts of extension into previous layer regions.
Other variations might use different deposition orders, different
amounts of over printing, or even quantities of deposition per
droplet. Still other variations might not use offset pixelling but
instead would use higher resolution pixels, possibly in combination
with deposition patterns yielding the right droplet density.
[0179] An additional interlacing technique combines: 1) feature
sensitivity, and 2) selective direction of scanning when exposing
object features. In this embodiment, cross-sectional geometry (e.g.
cross-sectional boundary information) from a current layer and
possibly cross-sectional geometry (e.g. cross-sectional boundary
information) from the immediately preceding layer would be used to
determine what the direction of scanning should be when exposing
different regions of the cross-section. For example, when exposing
the left most portion of a solid region of a cross-section it may
be advantageous to be scanning the head (i.e. the jet used for
exposing the line to be formed) from left to right if it is desired
that the droplet not bridge or not partially bridge any small gaps.
On the other hand, if it is desired that some bridging occur it may
be advantageous to ensure that scanning is in the opposite
direction. Similarly, when exposing the right most portion of a
solid region of the cross-section it may be advantageous to be
scanning from right to left (for no bridging) or from left to right
(for bridging). By controlling the scanning direction when
depositing boundary regions it can be ensured that horizontal
momentum of the droplets either do not contribute to bridging gaps
or enhance the bridging of gaps.
[0180] An example of the non-bridging technique is illustrated in
FIGS. 24a-24d. FIGS. 24a-d depict side views of two columns as
being formed and as cut in an XZ plane. The Z-direction is
perpendicular to the planes of the cross-sections and the
X-direction is the main scanning direction. Reference numeral 108
indicates the cross-section being formed and reference numerals
100, 102, 104, and 106 refer to previously formed cross-sections.
FIG. 24a depicts cross-section 108 with a broken line as no
material deposition has taken place. FIG. 24b indicates that the
scanning direction 110 is to the right and that droplets 112 are
deposited on the left most side of each column on a first pass.
FIG. 24c indicates that the scanning direction 124 is to the left
and that droplets 114 are deposited on the right most side of each
column in a second pass. FIG. 24d indicates that scanning can occur
in either direction 126 and that droplets 116, 118, 120, and 122
are deposited to complete the formation of the cross-section in a
third pass. As opposed to the illustrated three pass embodiment, a
two pass embodiment could be used wherein droplets 116, 118, 120,
and 122 may have been deposited to their respective locations
during one or both of the first or second passes when droplets 112
and 114 were deposited.
[0181] It is anticipated that the object could be relatively
reoriented (e.g. one or more rotations about the vertical axis)
with respect to the relative scanning direction of the print head
(i.e. jets) so that the edges of any desired cross-sectional
features can be exposed while relatively moving the print head in a
desired direction to enhance or decrease the probability of
bridging small gaps.
[0182] As noted above, if the orifice plate to working surface
distance is too small, droplets will have an elongated shape (i.e.
large aspect ratio) as they strike the working surface. In the case
of building with elongated droplets, it is anticipated that the
above indicated scanning directions for depositing on edges of
solid features might yield opposite results from those indicated
above. Other interlacing techniques might involve bi-directional
printing of adjacent raster lines or non-adjacent raster lines.
[0183] The above-described building techniques can be applied to
the formation of solid objects or in combination with other
techniques to the formation of partially hollow or semi-solid
objects. In an original design of an object, portions of the object
are supposed to be solid (i.e. be formed of solidified material)
and portions are supposed to be hollow (i.e. empty regions). In
actuality these intended hollow (or void) regions that are not
supposed to be part of the object, since by definition wherever
there is object there is supposed to be material. In the context of
the present invention a non-solid, hollow, or semi-solid object, is
an object built or to be built according to the teachings of some
preferred embodiments wherein a portion of what should be solid
object has been removed. A typical example of this might be the
hollowing out, partial hollowing out, or the honeycombing of what
was originally a solid structure of the object. These originally
solid structures are sometimes referred to as object walls
regardless of their spatial orientation. Some preferred build
styles form completely solid objects, while other build styles form
solid surface regions of the objects but hollowed out or partially
hollowed out interior regions. For example, the interior portions
of an object might be formed in a checkerboard, cross-hatched,
hexagonal, tiled, or honeycombed manner (these and other build
styles useful herein, as implemented in photo-based
stereolithography are described in the above referenced patents and
applications). The above non-solid deposition patterns can be
considered internal object support structures. As such the other
support structures described herein can also be used as internal
object support structures. Such non-solid objects would be lighter
in weight than their solid counterparts, they would use less
material, they might even be formed more quickly depending on the
details of the specific building parameters, and they might be
formed with less risk of encountering heat dissipation problems
since much less heated material is deposited during their
formation. These objects might be useful as investment casting
patterns due to the decrease in the possibility of cracking
molds.
Temperature Control
[0184] Additional object formation embodiments involve the
formation of the object wherein the partially formed object is
maintained within a desired temperature range as it is being formed
or is at least maintained such that the differential in temperature
across the part (or the gradient of temperature difference) is
small. If during object formation, the different portions of the
object are allowed to be at different temperatures, the object will
undergo a differential amount of shrinkage as it is cooled to room
temperature or as it is brought to its use temperature (the
temperature at which it will be put to use). This differential in
shrinkage could lead to the development of stresses within the
object and associated distortions or even fractures of the object.
It is preferred that temperature differential remain within a range
which is effective for maintaining object distortion within a
reasonable limit. The temperature differential across the object is
preferably maintained within a range of 20.degree. C., more
preferably within a range of 10.degree. C. and even more preferably
within a range of 5.degree. C. and most preferably within a range
of 3.degree. C. In any event, the desired temperature can be
estimated by taking into consideration the coefficient of thermal
expansion of the material and the differential in shrinkage (or
expansion) that would occur upon cooling (or heating) the formed
object to a uniform temperature. If the differential in shrinkage
results in an error outside a desired tolerance range, the above
mentioned ranges of temperatures can be adjusted.
[0185] In the formation of objects, the initial object data can be
scaled to take into account dimensional changes in the object that
will occur as the object is cooled from its jetting temperature
(about 130.degree. C. in the preferred embodiment) to its
solidification temperature (about 50.degree. C.-80.degree. C. with
a peak DSC energy transfer temperature of about 56.degree. C.) to
its build temperature (about 40.degree. C.-45.degree. C.) and
finally to its use temperature (e.g. room temperature--about
25.degree. C.). This scaling factor could be used to expand the
initial object design by an appropriate thermal shrinkage
compensation factor such that it would be appropriately sized at
its use temperature. It is further anticipated that one or more
geometry dependent or at least axes dependent shrinkage factors
could be used to at least partially compensate critical regions of
the object for expected variations in object temperature during
build up.
[0186] The temperature of the previously formed lamina and the
cooling rate of the lamina being formed have been found to be
important parameters for the formation of objects with reduced
distortion and in particular with reduced curl distortion.
Presently preferred materials undergo about 15% shrinkage when
cooled from their solidification temperature to room temperature.
This shrinkage provides a tremendous motivation force for causing
curl distortion, build up of internal stresses, and associated post
processing distortions (these distortions are described with regard
to photo-based stereolithography in the above referenced patents
and applications wherein many of the building techniques described
therein can be effectively utilized in the practice of SDM and TSL
in view of the teaching found in the instant application). It has
been found that if the object build temperature and in particular
if the temperature of the last formed layer is maintained at a
temperature above room temperature during the build process, curl
distortion will be reduced. It is preferred that the temperature of
the entire partially formed object be maintained above room
temperature and, more particularly, that its temperature remain
within a tight tolerance band due to the differential shrinkage
considerations discussed above.
[0187] For effective object formation, it is apparent that the
build temperature of the partially formed object must be maintained
below the melting point of the material. Additionally the build
temperature must be maintained below a temperature that allows the
solidified material to have sufficient shear and compressional
strength and even tensional strength (especially if sideways or
upside down object formation embodiments are used) to allow the
object to be accurately formed while experiencing the typical
forces associated with the build process (e.g. inertial forces
associated with accelerations the object will experience, drag
forces or vacuum forces associated with the planarizer and print
head contacting or passing close to the object, air pressure forces
associated with any air flow used to cool the object, and
gravitational forces on the object due to its own weight). Some of
these forces are dependent on the mass of the object and increase
with depth into the part. Thus, a slight negative temperature
gradient from higher to lower layers (i.e. decrease in temperature
from most recently formed layers to earliest formed layers) can
supply increasing strength in needed regions while simultaneously
allowing the latest formed layer or layers to be at a high enough
temperature to result in minimal curl and other distortions. One
might use a simple gravitational force calculation summed with an
inertial force calculation for one or more positions in the part
(based on the mass of the part and the Y-direction acceleration it
experiences) as an approximation of the minimum shear strength
needed from the solidified material. This in combination with an
empirical determination of the variation of material shear strength
with temperature can be used to estimate the approximate upper
build temperature limit for any position in the object. Of course
it is preferred that additional considerations be taken into
account, especially near the latest formed lamina of the object,
since dynamic thermal effects occur at the interface of the
partially formed object and the material being dispensed that
involve remelting phenomena and heat capacity phenomena which are
dependent on object geometry parameters, temperature differentials,
and cooling techniques. Thus, the actual overall maximum build
temperature will probably be lower than the above estimated
amount.
[0188] On the other hand, as noted above, curl and other
distortions can be significantly reduced by building at elevated
temperatures wherein the higher the temperature the less the
distortion. It is postulated that this reduction in distortion
results from a combination of the enhanced ability of the material
to flow at elevated temperatures and its lower ability to support
shear loads which allow some material redistribution to occur
thereby reducing stress which causes distortion. It is further
postulated that working near, at, or preferably above any solid
state phase change temperatures (e.g. crystallization temperature
or glass transition temperature) will result in the quickest and
potentially most significant reductions in stress and distortion.
Since these phase changes typically occur over a broad range,
various levels of benefit are postulated to occur depending on
where the working temperature is in within these ranges and the
process time allowed. Melting temperatures and/or solidification
temperatures and solid state transition temperatures can be
determined using Differential Scanning Calorimetry (DSC) techniques
which in turn can be utilized in determining appropriate build
temperature ranges. Additionally, appropriate build temperature
ranges can be determined empirically. It has been determined that
some benefit can be gained by working at any temperature above room
temperature and it is anticipated that the closer one moves to the
melting temperature and/or solidification temperature the more the
benefit. Thus, the working temperature range might be set as a
percentage of the distance along the temperature differential
between room temperature and melting or solidification temperature
or room temperature and the temperature of estimated minimum shear
strength. Alternatively, the working temperature may be selected to
be a temperature for which the material has a certain percentage of
its room temperature shear strength. For example it might be
desired to set the working (build) temperature such that the shear
strength is 75%, 50%, 25% or even 10% of its maximum room
temperature value.
Surface Finish Enhancement
[0189] Additional building embodiments useful for enhancing object
surface finish involve taking advantage of the aesthetically
pleasing, up-facing surfaces which result from the practice of
preferred SDM techniques. In these embodiments the number of
effective up-facing surfaces (e.g. the overall area) is increased
while the number of effective down-facing surfaces is reduced from
that defined by the original object design. This involves splitting
the object into two or more pieces and changing the orientation of
the separated pieces such that as many critical surfaces as
possible are made to be up-facing surfaces, vertical surfaces or
combined up-facing/vertical surfaces whereas no truly external
surfaces or only less critical surfaces remain as down-facing
surfaces. These separate object components are then built
independently of one another, each with the proper orientation.
Then supports are removed and the resultant components combined by
gluing, or the like, such that a complete object is formed
primarily from up-facing and vertical surface regions. If rough
surfaces are desired instead of smooth surfaces, the above
technique can be used to ensure that critical surfaces are formed
as down-facing surfaces. As an alternative the up-facing surfaces
which are to be roughened can simply be formed with supports
extending therefrom.
[0190] An example of this building technique is illustrated in
FIGS. 25a-e. FIG. 25a depicts the configuration of an object 60 to
be formed using SDM (i.e. the desired object design). If the object
is formed directly from this design, the object will be formed with
both up-facing features or surfaces (50, 52, and 54) and
down-features or surfaces (56 and 58). As discussed previously, the
formation of down-facing features requires the prior formation of a
support structure which acts as a working surface onto which
material forming the down-facing features is dispensed. After
object formation and removal of the supports, it has been found
that the down-facing surfaces are left with a rough and irregular
surface finish. If it is desired that the down-facing surface be
smooth, the object must undergo additional post-processing which
may require detailed sanding or filling.
[0191] FIG. 25b depicts the first step in the practice of the above
technique. This first step involves splitting the original or
desired object design into two or more components. The splitting is
performed so that all critical features of the object can be formed
as either vertical surfaces or up-facing surfaces (preferably as
up-facing surfaces and more preferably as up-facing surfaces which
do not have down-facing surfaces above them so that supports will
not be formed which start from and mar the up-facing surfaces).
Additional details about support formation and issues associated
therewith will be described further hereinafter. In the present
example, all surfaces 50, 52, 54, 56 and 58 are considered to be
critical and should be formed as up-facing surfaces.
[0192] FIG. 25b depicts object 60 being split into two portions 62
and 64. Portion 62 includes original outward facing features 50,
52, and 54 and new or temporary outward facing features 72 and 74.
Portion 64 includes original or desired outward facing features 56
and 58 and new or temporary outward facing features 72' and
74'.
[0193] FIG. 25c depicts the preferred orientation (right side up)
of portion 62 during formation so that surfaces 50, 52 and 54 are
formed as up-facing features. FIG. 25d depicts the preferred
orientation (upside down) of portion 64 during formation so that
surfaces 56 and 58 are formed as up-facing features. After
formation of each portion 62 and 64 the supports are removed and
temporary pairs of surfaces 72 & 72', and 74 & 74' are
prepared for mating. FIG. 25e depicts the joining of portions 62
and 64 to form object 60 wherein all critical outward facing
portions (i.e. original surfaces 50, 52, 54, 56 and 58) have good
surface finish.
Additional Build Styles
[0194] Other building styles might include one or more of the
following: 1) the use of higher resolution dispensing in the
scanning directions; 2) the use of a higher drop density per unit
area in forming down-facing skin surfaces than in forming interior
regions of the object; 3) the use of down-facing skin regions which
extend at least N layers (e.g., 5 to 10) above down-facing
surfaces; 4) the use of a higher drop density per unit area when
forming up-facing skin surfaces than in forming interior regions of
the object; 5) use of up-facing skin regions which extend for at
least N layers (e.g., 5 to 10) below an up-facing surface; 6) the
use of higher drop density per unit area when forming boundary
regions of an object than when forming interior regions, boundary
regions which extend at least L drop widths (e.g., 2 to 4) into the
interior of an object; and 7) forming interior regions of the
object through raster scanning and boundary regions through vector
scanning.
Support Styles:
[0195] The next portion of the application is primarily directed to
support formation. It should be appreciated, however, that as
supports are formed from deposited material, all the aforementioned
building techniques are applicable to the support building process.
Moreover, as will be appreciated, all aspects of the support
building process are applicable to object building as well.
[0196] Support structures must serve several needs which may be
opposing: 1) they preferably form a good working surface on which
to build object lamina and even successive support lamina; 2) they
are preferably easily removable from the down-facing surfaces they
support; 3) if they start from an up-facing surface of the object,
they are preferably easily removable therefrom; 4) when removed,
the supports preferably cause only minimal damage to down-facing
and up-facing surfaces and preferably have at least a tolerable to
good surface finish on those surfaces; 5) they preferably build up
at a reasonable rate per cross-section in the vertical direction
(e.g. Z-direction); 6) they are preferably formed using a minimal
number of passes per layer; and 7) their formation is preferably
reliable. A number of different support styles have been developed
or proposed which achieve different balances between these
needs.
[0197] To optimize building speed, vertical accumulation is
important and, as such, it is desirable to have supports build up
at approximately the same rate as the object. In particular, it is
preferred that the vertical accumulation of supports (e.g., from a
single pass per layer) be at least as great as a desired layer
thickness set by the use of the planarizer. The closer the support
-accumulation is to that of object accumulation, the thicker the
useable layer and the less material that will be removed during
planarization which thereby increases the efficiency of the
building process. For a given material and apparatus, the vertical
build up of material from different support and build styles can be
empirically determined, as described previously, by building test
parts for each deposition style or pattern using different layer
thicknesses (planarization levels) and thereafter measuring the
parts to determine when the build up of material lagged behind the
anticipated thickness as dictated by the number of layers deposited
and the expected layer thickness. From this information one can
either set the layer thickness (planarization level) to an
appropriate amount for a desired combination of build and support
styles or one can set the required support and build styles
necessary to achieve the desired layer thickness.
[0198] Some preferred support style embodiments emphasize speed of
formation, maintain easy removal, but leave rough surface finish in
regions where supports have been removed. This support style
involves the formation of solid columns which are separated by
small gaps. In particular, in a preferred system, data is supplied
at 300 pixels per inch in both the X and Y directions and the
object and supports are formed using four times ID overprinting in
the X direction (main scanning direction). Each layer of supports
includes three-by-three pixels zones where support material is to
be dispensed with the columns separated by two pixels zones of no
pixel defined deposition along the main scanning direction
(X-direction) and one pixel zone of no pixel defined deposition in
the secondary scanning direction (Y-direction). The data situation
defining these pixel zones is depicted in FIG. 15a. The "X's" in
the figure depict pixels which contain droplet data while the "O's"
in the figure depict pixels which contain "no droplet" data.
Squares 50 have been inscribed around the "X" zones so as to
highlight the shape of the deposition zones. However, due to the ID
overprinting in the X-direction, the two pixel gaps are actually
narrowed considerably (by almost one pixel width) when actual
deposition occurs. Thus, the actual resulting pattern of deposition
more closely approximates 4 by 3 pixel width (12-14 mils by 9-10
mils) columns, though with rounded comers, which are separated by 1
pixel width gap in both X and Y (3.3 mils). This situation is
approximately depicted in FIG. 18.
[0199] In the practice of building objects it has been found that
supports of the above configuration accumulate at approximately the
same rate as the object and thus a single pass of the head over
each drop site can be used for forming both supports and object on
each layer. It has also been found that the above support structure
is easily separable from the object but that a poor down-facing
surface finish results. Therefore, in terms of building speed, the
above style is preferred, but in terms of surface finish,
significant room for improvement remains.
[0200] A variant involves using multiple passes of the dispensing
head to form a support portion of a cross-section. Another
alternative involves periodically dispensing an extra support
cross-section in order to equalize vertical material accumulation
between supports and the object.
[0201] Another variant involves allowing support formation to lag
behind object formation by one or more layers to eliminate or
minimize planarization problems that can occur in the case where
fragile supports are being built. The problem is that the
planarizer can cause these supports to distort if support portions
of a cross-section are dispensed during the same pass or passes as
the corresponding object portion of the cross-section. By allowing
a lag of one or more layers to occur, excessive contact between the
supports and the planarizer can be avoided, and it is anticipated
that the resultant distortion of the supports will be
minimized.
[0202] Other column-like support structures are possible including
columns of different dimensions or shapes. For example, data
formatting and overprinting techniques could be combined to produce
physical columns of approximately a 3 by 3 pixel size (9-10 mils by
9-10 mils), 2-by-3 or 3-by-2 pixel size (these may result in less
vertical accumulation), 2-by-2 (6-7 mils by 6-7 mils) pixel size
(probable loss in vertical accumulation rate), 4 by 4 (12-14 mils
by 12-14 mils) pixel size (may be more difficult to remove and
cause further damage to object surfaces), or even larger sizes.
Other cross-sectionally shaped columns may also be used. These
might include more circularly shaped structures (e.g. octogonal or
hexagonal), cross-like structures, structures with different length
to width aspect ratios, or combinations of structures that can be
intermixed.
[0203] Other alternatives might include offsetting alternate
support columns in one or both of the main and subscanning
directions. For example, every other support column could be offset
in the secondary scanning direction by 1/2 the separation between
columns. This is depicted in FIG. 19. Wider spacing of support
columns is possible, particularly if some technique, such as arch
or branching supports are used to narrow the gap between the
support columns prior to encountering a down-facing surface of the
object. Two examples of arch-like supports are depicted in FIGS.
21a and 21b wherein different amounts of pixel offsetting (or at
least drop placement control) are used).
Branching Supports
[0204] As described at several locations herein above, some
preferred embodiments utilize supports that may be described as
branching supports. The arch-type supports discussed above are an
example of a type of branch support. Branching or branch-type
supports are support structures that are built such that portions
of some lamina extend outward in a cantilever manner from
solidified regions on the immediately preceding lamina. These
outward extensions may be based on identical (i.e. fixed) pixel
positions from layer to layer. Alternatively these outward
extensions may be based on fractional pixel width shifts in pixel
positions between some or all layers. Further alternatives may be
based on changing pixel patterns between some or all layers. Some
embodiments of branching supports produce more individual support
structures at a surface to be supported than the number of support
structures from which the branching supports originated at a lower
layer.
[0205] In addition to the various embodiments disclosed previously
(which in essence can be considered branching supports), FIG. 28a,
28b, 29a-e, 30a-m, 31a-c, 32a-d depict additional examples of
preferred branching support structures. FIG. 28a depicts a side
view of column supports 504, 506 & 508 starting at surface 500
and working up toward surface 502. These column supports are
connected one to another by branching elements 510, 512, 514 and
516. FIG. 28b depicts a side view of an embodiment of branching
type supports that work up from surface 500 toward surface 502. The
supports are shown to branch every two layers. In this two
dimensional view, some branching appears to occur in a two path
fork-like pattern while other branches simply branch out along a
single path. The same support structure depicted in FIG. 28b is
looked at from a different view in FIGS. 31a-c and 32a-d.
[0206] Other preferred branching patterns are illustrated in the
example of FIGS. 29a-e. FIGS. 29a-e depict top views of successive
branching cross-sections for a single support tree that uses X only
and Y only branches and results in a total of four support branches
from a single support trunk. FIG. 29a depicts a single support
structure that will be branched into a plurality of structures.
This single support structure may be called the "trunk" of the
support tree or structure. As will be made clear below, for ease of
data manipulation, the trunk can be considered to consist of four
separate but identical components which maintain their separate
identity, but may be Booleaned together to yield the scanning
pattern for any given layer. Of course in practice a real region to
be supported might require a plurality of these trunk elements
appropriately spaced from each other.
[0207] FIG. 29b depicts a first branching in the X direction. As
with the other Figures to follow, the hatched solid regions, as
depicted, represent the deposition regions for the instant
cross-section whereas the region(s) depicted with dashed lines
represent the immediately proceeding branch. This way of depicting
the deposition regions is done to make the registration between
branches clear. This first branching may occur after one or more
trunk layers are formed. As with other branches to be described
herein after in association with this figure and other figures to
follow, branching may extend the dispensed material out from
supported regions by a fraction of a pixel, a full pixel, or
multiple pixels depending on the drawing order used, the pixel
width as compared to the drop width, the number of identical layers
to be formed above the present layer (which can compensate for
imperfections in the present layer), ability of the material to be
partially unsupported, and the like. As with some of the other
branches, to be discussed herein after, this branching can be
looked at as a two way branch (i.e. one way in the positive X
direction and the other way in the negative X-direction) or as a
one-way branch of two or more initially overlapped components. As
will be seen from the description to follow, this first branch may
be considered a one-way branching of four initial components
wherein two components follow each branching direction. The actual
deposition of material from these four components may be based on a
Boolean union of the components so that multiple depositions over
overlapping regions are avoided.
[0208] FIG. 29c depicts the next branching of the tree wherein this
branching may initially occur one or more layers after the
branching depicted in FIG. 29b. This branching of object components
occurs in the same directions as seen in FIG. 29b.
[0209] FIG. 29d depicts two branchings in the Y-direction of each
of the two branches depicted in FIG. 29c. In concept, this may
again be considered a single branching in the Y direction of
separate components. The branching depicted in FIG. 29d is the
first branch which begins the separation process of all four
components.
[0210] FIG. 29e depicts a final branch for this example embodiment
wherein an additional Y-direction branch of each component is made.
These final branches can be used to support an object surface as
appropriate. If an object surface is located several layers above
these final branches, the structures (e.g. columns) of FIG. 29e can
be extended until the object surface is encountered. If the object
surface is not at the same level for all four branches the
individual columns or portions of columns can be extended as
necessary. This extension of support height is similar to other
preferred column support embodiments discussed herein and can
include the use of bridge layers and the like. Of course if
different configurations (e.g. shapes, positions, and the like) of
the four column branched support is desired, modifications (e.g.
modifications to branching order, branching directions, extension
amounts, number of layers between branches, and the like) to the
depicted embodiment can be made and will be apparent to one of
skill in the art in view of the teachings herein. The support trunk
depicted in FIG. 29a may initially be formed on a previous object
cross-section or initial substrate. Alternatively, the trunk may
begin on top of another support structure such as that depicted in
FIG. 28a. Furthermore, if multiple trees are to be used, branching
of the trees may or may not begin on the same layer and may or may
not result in each branch being formed after the same number of
layers. Selection of where to beginning branching and when to make
successive branches thereafter, may be based on the geometry of the
object to be formed. It may be desirable to have the final
branching pattern achieved, for a particular tree, several layers
before first encountering a surface to be supported (e.g.
down-facing object surface).
[0211] The branching routines performed in association with the
example embodiment illustrated in FIGS. 29a-29e may outlined in the
following table:
4 Component #1 Component #2 Component #3 Component #4 Build without
branching for a desired number of layers (FIG. 29a) Branch in the
Branch in the Branch in the Branch in the +X direction by +X
direction by -X direction by -X direction by desired desired
desired desired amount A amount A amount A amount A (FIG. 29b)
(FIG. 29b) (FIG. 29b) (FIG. 29b) Build without branching for a
desired number of layers Branch in the Branch in the Branch in the
Branch in the +X direction by +X direction by -X direction by -X
direction by desired desired desired desired amount A amount A
amount A amount A (FIG. 29c) (FIG. 29c) (FIG. 29c) (FIG. 29c) Build
without branching for a desired number of layers Branch in the
Branch in the Branch in the Branch in the +Y direction by -Y
direction by +Y direction by -Y direction by desired desired
desired desired amount A amount A amount A amount A (FIG. 29d)
(FIG. 29d) (FIG. 29d) (FIG. 29d) Build without branching for a
desired number of layers Branch in the Branch in the Branch in the
Branch in the +Y direction by -Y direction by +Y direction by -Y
direction by desired desired desired desired amount A amount A
amount A amount A Build without branching until a new support style
is implemented or until a surface of the object is encountered
[0212] As desired, the various parameters outlined in the above
table can be modified. For example the Branching amounts where
taken as an amount "A". As appropriate, this amount can vary with
different branching levels or it can even vary for different
components during the same branching level.
[0213] FIG. 30a-30m depict an analogous branching support
embodiment to that of FIGS. 29a-29e with the exception that the
single trunk depicted in FIG. 30a will give rise to 16 branches as
indicated in 10 FIG. 30m. For ease of understanding and possibly
implementation, the trunk shown in FIG. 30a can be considered as
consisting of 16 individual but identical components. Again, the
offset is performed along only one of either the X or the Y
directions during a given branching operation for a given
component. All the considerations noted above in describing FIGS.
29a-e can be applied to the example embodiment depicted in these
Figures as well as the example embodiments to follow.
[0214] FIGS. 31a-c depicts an additional example embodiment wherein
a single trunk, as depicted in FIG. 31a, is branched into four
elements, as depicted in FIG. 31c. This embodiment differs from
that in FIGS. 29a-29c in that branching occurs simultaneously in
both the X and Y directions. As illustrated, the extent of
branching is the same in both the X and Y directions but this
extent of branching could be varied between these directions.
[0215] FIG. 32a-32d continues the embodiment depicted in FIGS.
31a-31c to yield 16 separate branched supports. These Figures
further illustrate the Structure depicted in FIG. 28b wherein two
layers for each branch are depicted.
[0216] In other preferred embodiments other branching patterns are
possible. For example, instead of yielding rectangular arrays of
branched supports from individual trunks, as depicted in the above
described examples, hexagonal arrays, triangular arrays,
semi-circular arrays, or the like may be formed. If the achieved
patterns do not fit nicely together, it may be desirable to use a
mixture of patterns which are alternated in an appropriate fashion
to give a good fitting or meshing of the final support structures
such that a down-facing surface can be adequately supported. Other
preferred embodiments may use multiple trunks for supporting single
groups of branching supports.
[0217] It is anticipated that these branching support embodiments
might yield better down-facing surface than achieved with some of
the other preferred embodiments since it is believed that the final
support structures that contact the object will be more uniformly
spaced. As noted above, the branched support embodiments described
herein might be a part of a larger support structure or hybrid
support structure. Other modifications to the above embodiments
will be apparent to one of skill in the art after studying the
teachings herein.
[0218] If the geometry and direction sensitive interlacing
techniques described above are used it may be possible to build
smaller diameter and/or more closely spaced structures to provide a
better working surface while still providing reasonable vertical
accumulation rates.
[0219] In the preferred embodiment, deposited drop diameter is
approximately the same as the preferred pixel diameter (about
2.9-3.4 mils). in general, however, the pixel separation between
supports (e.g. separation between support columns) is less critical
than the separation relative to the falling drop diameter (e.g. 2
mils) and impacted (or deposited) drop diameter. Preferably the
horizontal spacing between supports (e.g. support columns) is less
than 6 drop diameters on the layer immediately preceding the layer
containing the down-facing surface to be supported. More
preferably, the spacing is less than 3 falling drop diameters, and
most preferably, the spacing is less than 1 to 2 falling drop
diameters.
[0220] It has been found useful to include periodic bridging
elements between the support columns to limit their ability to
shift from their desired XY positions as they grow in height.
Typically the smaller the diameter of the support columns the more
often bridging elements or layers are needed. These bridging
elements may extend one or more layers in height. In the preferred
embodiment, it has been found that a single layer (1-2 mils) of
bridging elements is not completely effective and that more than
five layers (5-10 mils) makes the overall support structure too
rigid. Thus, when using the preferred 3 by 3 pixel supports,
bridging layers are preferably between 2 layers (24 mils) and 5
layers (5-10 mils) in height and most preferably 3 layers (3-6
mils) in height. Furthermore, it has been found that the bridging
layers are preferably repeated every 75 mils to 2 inches, more
preferably every 100 to 300 mils, and most preferably every 100 to
200 mils. For use with other materials, building parameters, or
building conditions, formation and analysis of test parts can be
used to determine the effective bridge thickness and separations
thicknesses.
[0221] When bridging layers are periodically used they may bind all
support columns together or they may bind only a portion of them
together wherein the other columns were bound on a previous use of
bridging or will be bound on a subsequent use of bridging. In other
words, the bridging elements may form a solid plane of deposited
material or alternatively they may form only a partially solid
plane (e.g. a checkerboard pattern) which connects some of the
columns together. The support columns may or may not be shifted
from their previous XY positions when they are restarted after
formation of bridging layers.
[0222] Another preferred support structure which emphasizes easy
removal and good down-facing surface finish over speed of object
production is known as a checkerboard support. The cross-sectional
configuration of this support structure is depicted in FIG. 14.
Along each raster line, deposition occurs using every other pixel
(300 pixels/inch) and in adjacent raster lines the deposition
pixels are shifted along the line by one pixel width. One preferred
version of this support does not use ID overprinting, but can use
DD overprinting or multiple exposures to increase deposition per
layer. Without DD overprinting or multiple exposures, the layer
thickness when using this type of support in the preferred
embodiment is limited to under 0.4 to 0.5 mils, instead of the
approximately 1.3 mils obtainable with some preferred embodiments
described previously. Instead of using DD overprinting or multiple
exposures with these supports, it is possible to not use the
preferred ID overprinting of the object, and simply deposit
material in thinner layers (e.g. 0.3 to 0.5 mils per layer).
Overprinting of the object does not need to be utilized as the
extra material would simply need to be removed during the
planarization step. Since raster scanning is used and since the
speed of forming a layer is the same with or without overprinting,
build styles according to these techniques are approximately 3 to 4
times slower than equivalent build styles where four times
overprinting are used. Though there is a significant increase in
build time the improvement in surface finish may warrant its use
under certain circumstances.
[0223] When building checkerboard supports, regular use of bridging
layers is preferred (e.g. every 30 to 100 mils of z-height) to
ensure column integrity. The bridging layers should comprise a
sufficient number of layers to ensure their effectiveness (e.g.
about the same thickness of the bridging layers discussed above). A
drop-on/drop-off checkerboard pattern (in terms of drop width) is
where the solidified elements are 1 drop wide (deposition width),
and spacing between the center points of successive elements is
greater than 1 drop width but less than 2 drop widths.
[0224] Line supports (in terms of drop width) comprise line
elements which are approximately one impacted droplet diameter in
width, where the spacing between elements tangential to the
orientation of the lines is less than 1 drop width (i.e.
overlapping), while the spacing between elements perpendicular to
the line orientation is greater than 1 drop width. Preferably, the
spacing between elements perpendicular to the line orientation is
also less than 2 drop widths.
[0225] N-by-N column supports (in terms of pixels) are N-on,
preferably one or two-off in the main scan direction, and N-on, and
preferably 1-off in the index direction. The width of the columns
and spacing therebetween can be calculated based on a knowledge of
the pixel spacing, the drop diameter and any overprinting used. The
preferred spacing between deposited material in adjacent columns is
under one to two droplet diameters.
[0226] Another possible support style involves the use of solid or
periodically broken lines which are preferably less than 3 pixels
wide (less than 10 mils) and more preferably 1 to 2 pixels or less
in width (less than 3.3 to 6.6 mils) and are separated by 1 to 2
pixels or less of undeposited material (less than 3.3 to 6.6 mils).
These supports may run along the main scanning directions,
secondary scanning directions, or other directions. Another type of
support is curved line supports which follow the boundary of an
object. Alternatively, the support pattern can differ at different
areas of the cross-section. It can also be displaced from the
boundary of the object by N pixels (or drop widths) in the scan
direction, or M pixels (or drop widths) in the index direction.
[0227] Some other alternatives involve building supports from a
different material than used to form the surface or boundary
regions of the object. Other alternatives might use a different
support material only on one or more of the layers adjacent to the
object.
Hybrid Supports
[0228] Further types of support structures useful for Selective
Deposition Modeling are Hybrid Supports. In its simplest sense, a
hybrid support is a support structure that includes at least two
different types of support structures. Preferably, the structures
used in a hybrid support vary depending on the height of the
support and, more particularly, the structure at any given point
may depend on the distance from that point to an up-facing and/or
down-facing surface of the object. In other words, the support
structures are tailored to the most appropriate structure based on
the distance to the object. In an exemplary embodiment, the support
pattern is changed when the point is located a predetermined number
of layers (e.g., 4 to 9) below a down-facing surface. In another,
the drop density per unit area or drop density ratio (defined as
the drops to non-drops per unit area ratio) of the supports is
decreased as a down-facing surface is approached. In a variant of
these embodiments, one or more layers of shelving (or intermediate)
layers are used when transitioning from higher to lower drop
density ratio support structures.
[0229] In still another exemplary embodiment, the drop density
ratio is increased as an up-facing surface is left (e.g., 4 or more
layers away from an up-facing surface). In an optional variant of
this embodiment, one or more layers of shelving (intermediate)
layers are used when transitioning from lower to higher drop
density ratio support structures. It is also conceivable that
support structures could vary not just based on vertical distance
from the object but also based on horizontal distance as well. For
example, when horizontally bordering the object, a different type
of support may be useful than when some distance away from the
object.
[0230] An example Hybrid support is depicted from the side in FIG.
20. As shown, the structure extends from surface 23, which may be
the building platform, or which may be an up-facing surface of the
object being built, to support down-facing surface 24. As depicted,
the support structure consists of five components: (1) thin,
fiber-like columns 25 which contact surface 23 (if surface 23 is
not an up-facing surface of the object this component of the
support structure can be eliminated); (2) more massive columns 26
situated on top of the fiber-like columns 25; (3) intermediate
layers 27 (i.e. a final bridging layer); (4) thin, fiber-like
columns 28 situated on top of the intermediate layers and which
directly contact down-facing surface 24; and (5) bridging layers 29
which are used to fuse two or more of the massive columns together
and which are distributed at various places amongst the columns
26.
[0231] The thin columns 25 and 28 are both 1 pixel in cross-section
(3.3.times.3.3 mils) and form a "checkerboard" pattern as shown in
FIG. 14a. The result is a series of thin, fiber-like columns which
are spaced by 1 pixel from adjacent columns, and which easily
separate from surfaces 23 and 24. These are equivalent to the
checkerboard supports discussed above. Based on the one-pixel on,
one-pixel off deposition pattern of these supports the drop density
ratio is approximately 1. If the support does not start on an
up-facing surface of the object columns 25 can be skipped.
[0232] Columns 25 and 28 should be between 3 mils and 15 mils in
height and preferably about 4-6 mils in height. The height should
be held to a minimum since it is desired that these supports be
used in combination with an object which is being formed with 4
times ID overprinting and since when using a single pass on these
support structures without overprinting they accumulate at a much
slower rate than the object. On the other hand, it is desired that
these supports have some height since the needlelike elements tend
to melt down when the down-facing surface of the object is
dispensed onto them.
[0233] Columns 26 are 3.times.3 pixels in cross-section (9.9
mils.times.9.9 mils), and are spaced 2 pixels from adjacent columns
in the scanning direction, and 1 pixel from adjacent columns in the
index direction. These column supports are equivalent to the most
preferred supports discussed above. As discussed above, the primary
reason for the extra space in the main scanning direction is the
fact that these supports will receive 4 times overprinting. The
cross-sectional pattern formed by these columns is shown in FIG. 15
and 18. The result is a series of columns more massive than
fiber-like columns 25 and 28.
[0234] These columns, unlike the others, can be arbitrarily tall.
The reason is that the larger cross-sections of these columns allow
the columns to grow at about the same rate as the part itself
(about 1.3 mils/layer). As previously discussed, it is preferred
that bridges 29 be used to fuse adjacent ones of columns 26
together periodically to prevent "wandering" of these columns which
can occur after building up for some distance. The spacing of the
bridges is preferably in the previously discussed range.
[0235] The intermediate layers 27 represent an optional final layer
of bridging which can function as a transition between columns 26
and columns 28. The reason a transition layer is useful is that the
columns 28 are about the same size or smaller than the spaces
between the columns 26, with the result that without the transition
layers, the columns 28 may fall into these spaces. In one preferred
approach, intermediate layers as a whole would not be used and
instead careful placement of columns 28 on top of columns 26 would
occur or only the necessary portions of intermediate layers 27
would be used.
[0236] Preferably, if used, these intermediate layers are of
similar thickness to that of the previously discussed bridging
layers.
[0237] It should be appreciated that intermediate layers are not
needed between columns 25 and columns 26, because the columns 26
are larger in cross-section than the spacing between the columns
25. Accordingly, these larger columns can be built directly on top
of the smaller columns without the need of intermediate layers.
[0238] Other hybrid supports are possible that make other
combinations with the previously described support elements. The
hybrid and other support structures may also be used to form
internal portions of objects.
[0239] Additional alternatives exist for building supports. For
example, it is also possible to build the support from a material
which is different from that used to build the part. Another
possibility is to add a fluid such as water between the interstices
of the above described support structures in order to provide
additional support and also for aiding heat dissipation. In such an
approach, it is advantageous to use a fluid that has a greater
density than the building material. That will give buoyancy to the
drops of building material that fall between the interstices of the
columns. The material should also be chosen such that its surface
energy is matched to that of the building material in order to
prevent a meniscus forming between the fluid and the columns. An
example of such a material is a surfactant.
[0240] Another possibility is to shoot air jets upward between the
interstices of the columns. In this approach a heat dissipation
effect and buoyancy are possible. Another possibility is to fill
the interstices of a reduced number of column supports (e.g.
columns placed 0.1 to 1 inch or more apart) with particles.
Moreover, such particles could be formed from the building material
by allowing or causing the droplets to solidify before they reach
the working surface (such as by increasing the distance between the
dispensing head and the working surface), or by coating the
droplets before they land with a material that sublimes, i.e., goes
directly from a solid to a gas.
[0241] Supports preferably space the object from 50 to 300 mils
from the surface of the building platform. Alternatively, the
object may be directly built on the platform. In this alternative,
the platform may be covered with a flexible sheet material that
will allow easy separation of the object from the rigid platform
and then from the sheet material. An electric knife may be used to
separate the supports from the platform in which case it is
preferred that object be placed 150 to 300 mils above the platform
surface. A thin comb-like device with long teeth has been found
effective for removing the supports from the platform. In this
case, the thickness of the device dictates the required spacing
between the object and the platform, typically between 50 and 200
mils. The supports may be removed from the object by light rubbing,
brushing or by use of a small probe device such as a dental
tool.
[0242] Another variant involves incorporating the subject
embodiments into an integrated system, which includes a capability
for automatic part removal, and a cooling station. Other
alternatives involve using a low-melting, point metal as a building
material, a material filler, or different materials on different
raster lines or drop locations.
[0243] Further alternatives involve using larger drops for support
building than for part building. Another alternative involves the
use of powdered supports, which may be formed by allowing or
causing the droplets to solidify before they reach the working
surface, as described above.
[0244] Other embodiments might build up objects based on different
main-scanning direction orientations (e.g. Y or Z), other secondary
scanning direction orientations (e.g. X or Z) and other stacking
orientations (e.g. X or Y). Other embodiments might use other
absolute movement schemes to achieved the desired relative
movements between the object and print head. For example in some
embodiments absolute movement of the print head might occur in all
three directions, while in other embodiments absolute object
movement might occur in all three-directions. In still other
embodiments, non-Cartesian movement of the print head or object
might be used and jetting directions may vary from layer to layer
or portion of layer to portion of layer.
[0245] Though some embodiments have been described under headings
inserted in the application, these embodiments should not be
considered as pertaining only to the topic indicated by the header.
Furthermore, though headers were used to enhance the readability of
this specification, all disclosure relevant to the particular topic
recited by the header should not be considered as falling within
those single sections. All embodiments disclosed herein are useful
separately or in combination with other embodiments disclosed
herein.
[0246] While embodiments and applications of this invention have
been shown and described, it will be apparent to those skilled in
the art that many more modifications are possible without departing
from the inventive concepts herein. The invention, therefore, is
not to be restricted, except in the spirit of the appended
claims.
* * * * *